Instant judge leaves is not certain (4)
I believe the answer is:
iffy
'not certain' is the definition.
I can't judge whether this definition defines the answer.
'instant judge leaves' is the wordplay.
'instant' becomes 'jiffy' (synonyms).
'judge' becomes 'j' (abbreviation for judge).
'leaves' indicates named letters should be taken away.
'jiffy' with 'j' removed is 'IFFY'.
'is' is the link.
Can you help me to learn more?
(Other definitions for iffy that I've seen before include "Questionable - uncertain" , "Uncertain (colloq.)" , "not kosher" , "Doubtful (colloq.)" , "Unsure" .)