As a religion, one grand success (5)

I believe the answer is:

islam

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'as a religion' is the definition.
I can't tell whether this definition defines the answer.

'one grand success' is the wordplay.
'one' becomes 'i' (Roman numeral).
'grand success' becomes 'slam' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'i'+'slam'='ISLAM'

Can you help me to learn more?

(Other definitions for islam that I've seen before include "Mails to Muslim religion" , "Mails to great eastern religion" , "Creed" , "Religion based on Muhammad's teachings" , "Religion of the Koran" .)

I've seen this clue in the Evening Standard.

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