Crossword Genius

What He is mostly limiting? (7)

Ross

I believe the answer is:

atheism

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'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" .)

I've seen this clue in The Sunday Times.
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