Crossword Genius

He’s likely to be a Muslim? Most unlikely! (2,2)

Ross

I believe the answer is:

as if

'most unlikely' is the definition.

'he's likely to be a muslim?' is the wordplay.
I cannot quite understand how this works, but
'a' is within the answer.
'he' could be 'si' and 'si' is present in the answer.
A single letter 'f' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...

(Other definitions for as if that I've seen before include "I very much doubt it!" , "Yeah right!" , "You're joking!" , "Or though" , "Unbelievable!" .)

I've seen this clue in The Times.
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