Free a deserter imprisoned by soldiers (6)
I believe the answer is:
gratis
'free' is the definition.
(I know that free can be written as gratis)
'a deserter imprisoned by soldiers' is the wordplay.
'a deserter' becomes 'rat' (both can mean a traitor).
'imprisoned by' is an insertion indicator.
'soldiers' becomes 'gis' (GI is an informal term for a US soldier).
'rat' going inside 'gis' is 'GRATIS'.
(Other definitions for gratis that I've seen before include "On the house" , "Without payment" , "not charged" , "Without charge" , "Costing nothing, free" .)