One who operates within the house rules (4)
I believe the answer is:
user
'one who operates' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'within the house rules' is the wordplay.
'within' indicates a hidden word.
'USER' is hidden within 'the house rules'.
Can you help me to learn more?
(Other definitions for user that I've seen before include "Someone who exploits another" , "Substance taker" , "One operating something" , "Consumer" , "Drug-taker" .)