Child follows retreating gunners, firing irregularly (5)
I believe the answer is:
arson
'firing irregularly' is the definition.
(causing a fire)
'child follows retreating gunners' is the wordplay.
'child' becomes 'son' (male child).
'follows' means one lot of letters go next to another.
'retreating' says the letters should be written backwards.
'gunners' becomes 'RA' (Royal Artillery).
'ra' in reverse letter order is 'ar'.
'son' after 'ar' is 'ARSON'.
(Other definitions for arson that I've seen before include "Incendiarism" , "Incendiary crime" , "Fire-raising crime" , "Pyromaniac's crime" , "Criminal burning of property" .)