Free informant captured by US soldiers (6)
I believe the answer is:
gratis
'free' is the definition.
(synonyms)
'informant captured by us soldiers' is the wordplay.
'informant' becomes 'rat' (rat is a kind of informant).
'captured by' means one lot of letters goes inside another.
'us soldiers' becomes 'gis' (GI, nickname for US soldier).
'rat' going into 'gis' is 'GRATIS'.
(Other definitions for gratis that I've seen before include "Free, at no cost" , "Without charge" , "Free (Latin)" , "not charged" , "Free, costs nothing" .)