US soldiers crossing desert, free (6)
I believe the answer is:
gratis
'free' is the definition.
(I know that free can be written as gratis)
'us soldiers crossing desert' is the wordplay.
'us soldiers' becomes 'gis' (GI, nickname for US soldier).
'crossing' means one lot of letters goes inside another.
'desert' becomes 'rat' (abandon treacherously).
'gis' enclosing 'rat' is 'GRATIS'.
(Other definitions for gratis that I've seen before include "not charged" , "On the house" , "Free, costs nothing" , "Without payment" , "Costing nothing, free" .)