Free renegade held by US soldiers (6)
I believe the answer is:
gratis
'free' is the definition.
(I know that free can be written as gratis)
'renegade held by us soldiers' is the wordplay.
'renegade' becomes 'rat' (I've seen this before).
'held by' indicates putting letters inside.
'us soldiers' becomes 'gis' (GI, nickname for US soldier).
'rat' inserted inside 'gis' is 'GRATIS'.
(Other definitions for gratis that I've seen before include "Free, at no cost" , "Free of charge" , "Costing nothing, free" , "Without charge" , "Free, costs nothing" .)