Servicemen covering defect on the house (6)
I believe the answer is:
gratis
'on the house' is the definition.
(I've seen this in another clue)
'servicemen covering defect' is the wordplay.
'servicemen' becomes 'gis' (plural of GI - private US soldier).
'covering' means one lot of letters goes inside another.
'defect' becomes 'rat' (ratting is a kind of defecting).
'gis' placed around 'rat' is 'GRATIS'.
(Other definitions for gratis that I've seen before include "Without payment" , "Without charge" , "Free, costs nothing" , "Costing nothing, free" , "Free of charge" .)