Judge became vigilant, having missed opening crack (4)
I believe the answer is:
joke
'crack' is the definition.
(I've seen this in another clue)
'judge became vigilant having missed opening' is the wordplay.
I cannot really understand how this works, but
'judge' could be 'j' (abbreviation for judge) and 'j' is located in the answer.
The remaining letters 'oke' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for joke that I've seen before include "Crack in a cracker" , "Humorous remark" , "someone inadequate" , "Funny story" , "Quip, witticism" .)