Almost join detachment (4)
I believe the answer is:
unit
'detachment' is the definition.
(I've seen this before)
'almost join' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'join' becomes 'unite' (synonyms).
'unite' with its last letter taken away is 'UNIT'.
(Other definitions for unit that I've seen before include "Detachment of soldiers" , "Small group" , "Part of a kit" , "squad" , "Self-contained section" .)