He's paid to mind someone else's business (8)
I believe the answer is:
watchman
'he's paid to mind someone else's business' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he's paid to mind someone' is the wordplay.
I cannot really understand how this works, but
'someone' could be 'watch' (watch is a kind of someone) and 'watch' is found in the answer.
'he' could be 'man' (I've seen this in another clue) and 'man' is present in the answer.
No letters remain.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for watchman that I've seen before include "'Go Set a --', Harper Lee novel" , "Sentinel" , "fellow on duty?" , "Night guard" , "Security guard" .)