Free the deserter the soldiers captured (6)
I believe the answer is:
gratis
'free' is the definition.
(synonyms)
'deserter the soldiers captured' is the wordplay.
'deserter' becomes 'rat' (both can mean a traitor).
'the soldiers' becomes 'GIs' (GI is an informal term for a US soldier).
'captured' means one lot of letters goes inside another.
'rat' placed inside 'gis' is 'GRATIS'.
'the' is the link.
(Other definitions for gratis that I've seen before include "On the house" , "Free of charge" , "Free, costs nothing" , "not charged" , "Free (Latin)" .)