Join European attached to small military force (5)
I believe the answer is:
unite
'join' is the definition.
(synonyms)
'european attached to small military force' is the wordplay.
'european' becomes 'E' (abbreviation e.g. EU).
'attached to' is a charade indicator (letters next to each other).
'small military force' becomes 'unit'.
'e' put after 'unit' is 'UNITE'.
(Other definitions for unite that I've seen before include "Join together, merge" , "pool resources" , "Wed?" , "Combine - join" , "Join together, become one" .)