One is a leader in Mosque (4)

I believe the answer is:

imam

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'leader in mosque' is the definition.
'imam' can be an answer for 'leader' (imam is a kind of leader). I am not sure about the 'in mosque' bit.

'one is a' is the wordplay.
'one' becomes 'i' (Roman numeral).
'is a' becomes 'mam' (I can't explain this - if you can you should give a lot more credence to this answer).
'i'+'mam'='IMAM'

Can you help me to learn more?

(Other definitions for imam that I've seen before include "Islamic minister" , "Prayer leader in a mosque" , "Man leading prayers in a mosque" , "Ma, I'm off to find religious Muslim" , "Muslim cleric" .)

I've seen this clue in The Telegraph.

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