Object reserved briefly (5)
I believe the answer is:
demur
'object' is the definition.
(to demur is to object to something)
'reserved briefly' is the wordplay.
'reserved' becomes 'demure' (I've seen this in another clue).
'briefly' means to remove the last letter.
'demure' with its final letter taken off is 'DEMUR'.
(Other definitions for demur that I've seen before include "Dissent, object" , "Cavil, dispute" , "Take exception (to)" , "Hesitate from uncertainty or object" , "Hesitation" .)