It's no fun as a rule for him (7)
I believe the answer is:
ascetic
'a rule for him' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'it's no fun as' is the wordplay.
I cannot really see how this works, but
'as' is within the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for ascetic that I've seen before include "Person who lives severely simple life" , "Leading a severely simple life without worldly pleasures" , "Hermit" , "reclusive" , "recluse" .)