Judge uncertain for a moment (5)
I believe the answer is:
jiffy
'moment' is the definition.
(I know that jiffy is a type of moment)
'judge uncertain' is the wordplay.
'judge' becomes 'J' (abbreviation for judge).
'uncertain' becomes 'iffy' (both can mean dubious).
'j'+'iffy'='JIFFY'
'for a' is the link.
(Other definitions for jiffy that I've seen before include "Very short moment - type of bag" , "Brief moment" , "Moment (slang)" , "Two shakes of a lamb's tail?" , "Split second" .)