He runs for a living (7)
I believe the answer is:
manager
'he runs for a living' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he runs for' is the wordplay.
'he' becomes 'man'.
'runs for' becomes 'ager' (I can't explain this - if you can you should give a lot more credence to this answer).
'man'+'ager'='MANAGER'
Can you help me to learn more?
(Other definitions for manager that I've seen before include "Executive or controller" , "head of department" , "who sits on the bench?" , "Someone in control (or who copes?)" , "Person in charge of staff" .)