Free turncoat held by US soldiers (6)
I believe the answer is:
gratis
'free' is the definition.
('gratis' can be a synonym of 'free')
'turncoat held by us soldiers' is the wordplay.
'turncoat' becomes 'rat' (I've seen this in other clues).
'held by' is an insertion indicator.
'us soldiers' becomes 'gis' (GI, nickname for US soldier).
'rat' placed into 'gis' is 'GRATIS'.
(Other definitions for gratis that I've seen before include "Free (Latin)" , "On the house" , "Free, costs nothing" , "Free of charge" , "Free, at no cost" .)