He'll have to do (5)
I believe the answer is:
actor
'he'll have to do' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he'll have to' is the wordplay.
'he'll' becomes 'acr' (I can't explain this - if you can you should give a lot more credence to this answer).
'have' indicates putting letters inside.
'acr' going around 'to' is 'ACTOR'.
Can you help me to learn more?
(Other definitions for actor that I've seen before include "Gielgud, say" , "Performer in drama" , "One in a play" , "ham" , "Person performing on stage or screen" .)