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