Free the deserter the soldiers captured (6)
I believe the answer is:
gratis
'free' is the definition.
('gratis' can be a synonym of 'free')
'the deserter the soldiers captured' is the wordplay.
'the deserter' becomes 'rat' (I've seen this before).
'the soldiers' becomes 'gis' (I've seen this in other clues**).
'captured' indicates putting letters inside.
'rat' going within 'gis' is 'GRATIS'.
(Other definitions for gratis that I've seen before include "On the house" , "Free of charge" , "Free, costs nothing" , "not charged" , "Free (Latin)" .)