One who fires back with regard to legs (6)
I believe the answer is:
sniper
'one who fires' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'back with regard to legs' is the wordplay.
'back' is a reversal indicator.
'with regard' becomes 're' ('re' means regarding or about).
'to legs' becomes 'pins' (pin is a kind of leg).
're'+'pins'='repins'
'repins' in reverse letter order is 'SNIPER'.
Can you help me to learn more?
(Other definitions for sniper that I've seen before include "frets about having lost sixth place" , "Concealed gunner" , "Single marksman in concealed place" , "Hidden rifleman" , "Military marksman" .)