This can be fixed.
Sorry for length, but I actually disagree that some people aren't made for D&D. Any group of friends can play D&D, but you do need to customize it so it works.
You need to find out what X likes about playing D&D. Some players just enjoy being chaotic and "everything goes". If you push them into the railroad of an existing storyline where everybody must be a serious-minded Indiana Jones who enjoys solving complex puzzles and making strategic decisions, then it just won't work. But this is D&D, and the DM and the players must work together.
I myself DM some chaotic players (yes, the players are chaotic, and they obviously play chaotic characters). I run a big sandbox game, and players can summon weird monsters at the most inconvenient places. I never prepare much storyline because you cannot herd cats. It's a lot of improvisation for me as DM, so I work on the world in which we play (locations, NPCs and big developments in the world that change it) but I don't work on storylines for the main characters. The players decide on that themselves most of the time.
We did however agree (as players and DM) that the characters would accept whatever consequences followed their random behavior. Their often hilarious decisions have shaped the story in ways that I would never have imagined.
It's important to first agree what kind of campaign you will play. Also, you must agree what kind of team you will form. But chaotic play is definitely possible. You can play the Suicide Squad (all chaotic), you can play the A-team (one chaotic player (Murdoch) with three less chaotic characters) or you can play something else. But it's important that all players accept the gameplay of the others.
Session zero
To me, it sounds like you should stop whatever campaign you currently play and have a real session zero. That means all people (players and DM) must be willing to change their plans. Better yet, you shouldn't have any detailed plans yet. Just explain some basic ideas at the table:
Example:
Player A: I want to play a gritty fighter, serious and dark, possibly a bit roguish too (multiclassing later?)
Player B: I want to play a calm druid, wise and knowledgeable.
Player X: I want to play a wacky clown. If it must be PHB, then it's probably a bard. I carry two pots that I beat with a pink stick for music.
DM: I may want to play Stradh.
Then you discuss why on earth a fighter a druid and a clown would want to hang out together and trust each other with their lives? If the fighter or the druid realize (in character) that they would not want to hang out with a clown, then how can you change it so that they would? Likewise, why would this clown hang out with these serious other characters? Is there a more serious undertone hidden? If you can work this out, then great. If not, maybe someone wants to change? Can you find another gritty character or a wise nature lover that would hang out near the asylum?
And the DM must check if the chosen adventure fits this party. Will they accept a railroad campaign, or do they need freedom to chase random (side-)quests? Maybe a clown and a druid don't fit in a campaign that's exclusively in the Underdark? Is there a plan B for the campaign setting?
Session zero may last hours, and may remain inconclusive after 1 day. I also recommend to play a real short mini-quest once you think you have chosen the setting and characters. Just hunt down some monster in a 1-2 hour session and see how everybody interacts... and then sit down out-of-character one more time to check how that went.
Don't give up just because one of your players doesn't fit into the current campaign.
