What He is mostly limiting? (7)
I believe the answer is:
atheism
'what he is mostly limiting?' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'he is mostly limiting?' is the wordplay.
'mostly' becomes 'atm' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'limiting?' is an insertion indicator.
'he'+'is'='heis'
'heis' placed within 'atm' is 'ATHEISM'.
Can you help me to learn more?
(Other definitions for atheism that I've seen before include "Owed" , "Non-belief in God" , "displaying opposition to religion" , "Disbelief in gods" , "Sceptic's position" .)