Messenger or porter, say, carrying letters (7)
I believe the answer is:
apostle
'messenger' is the definition.
(I've seen this before)
'porter say carrying letters' is the wordplay.
'porter say' becomes 'ale' (porter is a kind of ale).
'carrying' is an insertion indicator.
'letters' becomes 'post' (I've seen this before).
'ale' going around 'post' is 'APOSTLE'.
'or' acts as a link.
(Other definitions for apostle that I've seen before include "'Matthew or Mark, say (7)'" , "Originally, one sent to preach the gospel" , "Missionary or evangelist" , "One of Christ's 12 original disciples" , "One of the biblical twelve (7)well don" .)