He'll do, in part (5)
I believe the answer is:
actor
'part' is the definition.
(thesaurus)
'he'll do' is the wordplay.
I cannot really understand how this works, but
'do' could be 'act' (I've seen this before) and 'act' is located in the answer.
The remaining letters 'or' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'in' acts as a link.
Can you help me to learn more?
(Other definitions for actor that I've seen before include "Thesp" , "Gielgud, say" , "One taking part" , "I pretend" , "For example, Colman" .)