Uncertain moment, dropping head (4)
I believe the answer is:
iffy
'uncertain' is the definition.
(both can mean dubious)
'moment dropping head' is the wordplay.
'moment' becomes 'jiffy' (I've seen this before).
'dropping head' 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 "Problematic" , "Dubious (colloq.)" , "Unsure" , "Dubious, risky (inf.)" , "Doubtful, not definite" .)