He'll do (5)
I believe the answer is:
actor
'he'll' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'do' is the wordplay.
I cannot quite understand how this works, but
'do' could be 'act' (I've seen this before) and 'act' is found within the answer.
The remaining letters 'or' 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 actor that I've seen before include "one to take on roles" , "Cruise, e.g." , "Stage player" , "Thespian, stage performer" , "One taking part" .)