If not resting, he pretends (5)
I believe the answer is:
actor
'if not resting he pretends' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
This is all the clue.
Can you help me to learn more?
(Other definitions for actor that I've seen before include "Stage-player" , "Person performing on stage or screen" , "Thesp" , "Cruise, e.g." , "One in a play" .)