Mails sorted for the Muslim world (5)
I believe the answer is:
islam
'the muslim world' is the definition.
(I've seen this in another clue)
'mails sorted' is the wordplay.
'sorted' is an anagram indicator.
'mails' with letters rearranged gives 'ISLAM'.
'for' acts as a link.
(Other definitions for islam that I've seen before include "Mohammed's religion" , "Religion of the Koran" , "Mails to great eastern religion" , "Adherents of a religion" , "Faith with five pillars" .)