Instant judgment initially dubious (5)
I believe the answer is:
jiffy
'instant' is the definition.
('jiffy' can be a synonym of 'instant')
'judgment initially dubious' is the wordplay.
'initially' says to take the initial letters.
'dubious' becomes 'iffy' (I've seen this before).
The first letter of 'judgment' is 'j'.
'j'+'iffy'='JIFFY'
(Other definitions for jiffy that I've seen before include "Split second" , "Very short moment - type of bag" , "Two shakes of a lamb's tail?" , "An instant" , "Moment (slang)" .)