Gunners returned to fix light (3,4)
I believe the answer is:
arc lamp
'light' is the definition.
(arc lamp is a kind of light)
'gunners returned to fix' is the wordplay.
'gunners' becomes 'ra' (Royal Artillery).
'returned' is a reversal indicator.
'to fix' becomes 'clamp' (clamping is a kind of fixing**).
'ra' in reverse letter order is 'ar'.
'ar'+'clamp'='ARC-LAMP'
(Other definitions for arc lamp that I've seen before include "Electric light source" , "Source of bright light" , "one of the bright lights" , "Intense illuminator" .)