When resting, he may well be keen to work (5)
I believe the answer is:
actor
'when resting he' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'well be keen to work' is the wordplay.
'well be keen' 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).
'work' becomes 'act' (synonyms).
'or' put after 'act' is 'ACTOR'.
'may' is the link.
Can you help me to learn more?
(Other definitions for actor that I've seen before include "Films/stage worker" , "One taking part" , "Gielgud, say" , "One on stage" , "eg Anthony Hopkins" .)