Uncertain moment heading off (4)
I believe the answer is:
iffy
'uncertain' is the definition.
(both can mean dubious)
'moment heading off' is the wordplay.
'moment' becomes 'jiffy' (I've seen this before).
'heading off' means to remove the first letter.
'jiffy' with its first letter taken off is 'IFFY'.
(Other definitions for iffy that I've seen before include "Doubtful (colloq.)" , "Doubtful, not definite" , "Sixth (RU) Nation" , "Questionable - uncertain" , "Informally dubious or uncertain" .)