Second judge uncertain (5)
I believe the answer is:
jiffy
'second' is the definition.
(thesaurus)
'judge uncertain' is the wordplay.
'judge' becomes 'j'.
'uncertain' becomes 'iffy' ('iffy' can be similar in meaning to 'uncertain').
'j'+'iffy'='JIFFY'
(Other definitions for jiffy that I've seen before include "Brief moment" , "Very short moment - type of bag" , "Two shakes of a lamb's tail?" , "Moment (slang)" , "Split second" .)