He's in the firing line (6)
I believe the answer is:
stoker
'he's in the firing line' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he's in the' is the wordplay.
I cannot quite understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
'he' could be 'ker' and 'ker' is found in the answer.
The remaining letters 'so' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for stoker that I've seen before include "Bram ...... wrote 'Dracula'" , "One feeding furnace" , "Fireman (on steam train)" , "One tends to fire" , "Irish author of Dracula, d. 1912" .)