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