In jest, to some extent, they’re OK (5)
I believe the answer is:
'in jest to some extent' is the definition.
I can't judge whether this definition defines the answer.
'ok' is the wordplay.
I cannot really understand how this works, but
'ok' is within the answer.
This may be the basis of clue (or it may be nonsense).
'they're' acts as a link.
Can you help me to learn more?
(Another definition for jokes that I've seen is " Gags".)