Free deserter caught by US soldiers (6)
I believe the answer is:
gratis
'free' is the definition.
(synonyms)
'deserter caught by us soldiers' is the wordplay.
'deserter' becomes 'rat' (both can mean a traitor).
'caught by' indicates putting letters inside.
'us soldiers' becomes 'gis' (GI, nickname for US soldier).
'rat' going within 'gis' is 'GRATIS'.
(Other definitions for gratis that I've seen before include "Free of charge" , "On the house" , "Free (Latin)" , "Without charge" , "not charged" .)