clearstream
(He, Him)
The Alert character is reacting to a sense of danger. I'd likely grant them a Perception check if they called for one. Either way, they get to move and act or ready an action. To quote Professor Farnsworth, "What do you want? A floral dress?"So, then, to circle back, there's a problem with letting the guy that failed his perception check know that there's a bad guy over there at the start of combat, but there's no problem in letting the guy that failed his perception check know that a combat has started, just without any actual clues or information, yeah? So, then, your problem isn't that you can't see a way to provide information to the Alert player, it's just some strange idea that the game is made better by making that player jump through hoops blindly for your amusement for daring to roll a higher initiative than you?
Because, and let be honest, if the initial attack of the hidden bad guy can kill the player, the blind fumbling of acting without information will be highly unlikely to change that outcome. So, acting first with no information becomes that guessing game where you hope you picked the right thing before the DM kills you. Poor play. So, then, if the attack can't outright kill the player, then they are actually more effective going second, especially if their attack can kill or significantly impede the enemy. So the usual outcome is that forcing the player to go first isn't actually helping the player, it's just ensuring that you get your ambush off as you scripted it. And that's the problem, you're forcing the game into a pretzel so that you can have your moment. As a DM, that's a tempting trap, but, here's the thing, you don't need to do this. You don't need to abuse the player and make them dance arbitrarily because you will get your moments. It might not be this one, because of the dice, but it will come and be sweeter because you haven't had to play the screw-u game with your players. If you really need the thrill of your players not being able to be effective because of their build choices, just narrate at them. Don't waste their time pretending to have a game.
Do you feel like 5e needs the Delay action? Or something else?