Female US soldier joining both sides (4)
I believe the answer is:
girl
'female' is the definition.
(girl is a kind of female)
'us soldier joining both sides' is the wordplay.
'us soldier' becomes 'gi' (I've seen this in other clues).
'joining' means one lot of letters go next to another.
'both sides' becomes 'rl' (I've seen this in other clues).
'gi'+'rl'='GIRL'
(Other definitions for girl that I've seen before include "Maidservant" , "Lass" , "Maid" , "Young lady" , "A young female" .)