Gunners go back to the lines for missiles (6)
I believe the answer is:
arrows
'missiles' is the definition.
(arrow is a kind of missile)
'gunners go back to the lines' is the wordplay.
'gunners' becomes 'ra' (Royal Artillery).
'go back' says the letters should be written backwards.
'to the lines' becomes 'rows' (row is a kind of line**).
'ra' back-to-front is 'ar'.
'ar'+'rows'='ARROWS'
'for' is the link.
(Other definitions for arrows that I've seen before include "Bolts that are shot" , "Archers shafts" , "Missiles for the bowman" , "Pointed missiles" , "Direction indicators" .)