Soldiers detaining deserter without charge (6)
I believe the answer is:
gratis
'without charge' is the definition.
(thesaurus)
'soldiers detaining deserter' is the wordplay.
'soldiers' becomes 'gis' (I've seen this in other clues**).
'detaining' is an insertion indicator.
'deserter' becomes 'rat' (I've seen this before).
'gis' going around 'rat' is 'GRATIS'.
(Other definitions for gratis that I've seen before include "not charged" , "Free of charge" , "Without charge" , "Without payment" , "Free (Latin)" .)