Instant when Jack leaves is unsure (4)
I believe the answer is:
iffy
'is unsure' is the definition.
(I've seen this before)
'instant when jack' is the wordplay.
I cannot quite understand how this works, but
'when' could be 'if' and 'if' is present in the answer.
The remaining letters 'fy' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'leaves' acts as a link.
Can you help me to learn more?
(Other definitions for iffy that I've seen before include "Unsure" , "Not certain, doubtful" , "Doubtful (colloq.)" , "Dubious, risky (inf.)" , "Uncertain (colloq.)" .)