D&D 5E Protection from Zone of Truth

[Quickleaf]

Out of curiosity [-]my BBEG[/-] - asking for a friend - would like to know what you think is the most easily available protection from this pesky Zone of Truth spell the opposing cleric prepares tomorrow...

(It would be swell if it were available on short notice... I do not have much time to prepare. They are already on my doorstep!)

The best defense against zone of truth?

The #1 answer is a well-designed genre-savvy story that acknowledges the existence of such magic in the world. For example, this ensures multiple suspects, everyone having something they feel guilty or could be incriminated about (so silence cannot be interpreted as guilt...or if it is there's plenty of opportunity for the players to make a mistake...which should have consequences), interrogated creatures firmly exploiting that grey middle ground between "lie" and "truth", and having multiple layers to any mysteries (i.e. so they aren't one-shotted by spells like this).

Another option more readily available than glibness is simply casting dispel magic on the area where the zone of truth enchantment has been cast. Or simply leaving the area works.

 

[Saeviomagy]

Strictly speaking, sentence fragments cannot be lies. Answer yes or no to questions and leave it at that.
Waffle, as others have said.
Cast modify memory on yourself to get rid of the incriminating memories OR fabricate alternative ones.
Write a short story. When the PCs come to the door, frame the conversation so that it's about your short story. "What did you want to know about my story?". Similarly you could start the conversation by talking about a book and then answer all subsequent questions as if they were about the book. In short, deliberately introduce confusion into the conversation.
Speak only in possibilities and questions. "It's possible that I was home all afternoon". "Why would I kill the mayor?". "I'd have to be pretty stupid to steal that gem and not expect some adventurers to come to my home asking about it while under a zone of truth".

As for what zone of truth is useful for? Swearing oaths and having some expectation that they will be followed. Confirming things that the PCs already suspect or know "Repeat after me: I am Duke Lyington. I was born to Lady Eliza Lyington and Lord Marfoy Lyington. I have lived in this city all my life." etc etc.

In short: don't use it to detect lies. Use it to confirm truths.