He'll just have to 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.
'just have to do' is the wordplay.
'just have' becomes 'or' (I can't justify this - if you can you should believe this answer much more).
'to' says to put letters next to each other (I've seen this in other clues).
'do' becomes 'act' (I've seen this before).
'or' put after 'act' is 'ACTOR'.
Can you help me to learn more?
(Other definitions for actor that I've seen before include "Player - performer" , "Dramatic artist" , "ham" , "Thespian, stage performer" , "Performer in drama" .)