Free traitor imprisoned by soldiers (6)
I believe the answer is:
gratis
'free' is the definition.
('gratis' can be a synonym of 'free')
'traitor imprisoned by soldiers' is the wordplay.
'traitor' becomes 'rat' (I've seen this before).
'imprisoned by' indicates putting letters inside.
'soldiers' becomes 'gis' (I've seen this in other clues).
'rat' going inside 'gis' is 'GRATIS'.
(Other definitions for gratis that I've seen before include "not charged" , "Free (Latin)" , "Free, at no cost" , "Free, costs nothing" , "Without charge" .)