Briefly join detachment (4)
I believe the answer is:
unit
'detachment' is the definition.
(I've seen this before)
'briefly join' is the wordplay.
'briefly' means to remove the last letter.
'join' becomes 'unite' (uniting is a kind of joining).
'unite' with its last letter taken off is 'UNIT'.
(Other definitions for unit that I've seen before include "Individual thing; one" , "Subdivision of a military group" , "One - kitchen cabinet" , "A considerable number of soldiers" , "Entity, system; component group" .)