Sack the fellow - he's burning to fight! (7)
I believe the answer is:
fireman
'he's burning to fight' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'sack the fellow' is the wordplay.
'sack' becomes 'fire' (both can mean to dismiss from a job).
'the fellow' becomes 'man' (fellow can mean a man).
'fire'+'man'='FIREMAN'
Can you help me to learn more?
(Other definitions for fireman that I've seen before include "Railway driver's assistant" , "Name fir (anag.)" , "who does a rushed job on tender?" , "Steam engine worker" , "Red engine driver" .)