Soldiers locking up deserter without charge (6)
I believe the answer is:
gratis
'without charge' is the definition.
(thesaurus)
'soldiers locking up deserter' is the wordplay.
'soldiers' becomes 'gis' (I've seen this in other clues**).
'locking up' indicates putting letters inside (I've seen this in other clues).
'deserter' becomes 'rat' (I've seen this before).
'gis' placed around 'rat' is 'GRATIS'.
(Other definitions for gratis that I've seen before include "Free, at no cost" , "Without charge" , "On the house" , "Free (Latin)" , "Costing nothing, free" .)