Deserter held by soldiers without charge (6)
I believe the answer is:
gratis
'without charge' is the definition.
(thesaurus)
'deserter held by soldiers' is the wordplay.
'deserter' becomes 'rat' (I've seen this before).
'held by' indicates putting letters inside.
'soldiers' becomes 'gis' (I've seen this in other clues).
'rat' placed into 'gis' is 'GRATIS'.
(Other definitions for gratis that I've seen before include "Free of charge" , "Without payment" , "Free (Latin)" , "Costing nothing, free" , "On the house" .)