Briefly, do bit (4)
I believe the answer is:
part
'bit' is the definition.
(both can mean a piece of something)
'briefly do' is the wordplay.
'briefly' means to remove the last letter.
'do' becomes 'party' (I've seen this before).
'party' with its last letter removed is 'PART'.
(Other definitions for part that I've seen before include "Constituent - something to play?" , "Some; break" , "Portion of a whole" , "Region" , "Element" .)