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 present in the answer.
The remaining letters 'or' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
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" .)