Porter collecting mail for messenger (7)
I believe the answer is:
apostle
'messenger' is the definition.
(I've seen this before)
'porter collecting mail' is the wordplay.
'porter' becomes 'ale' (I've seen this in other clues).
'collecting' means one lot of letters goes inside another.
'mail' becomes 'post' (I've seen this before).
'ale' going around 'post' is 'APOSTLE'.
'for' is the link.
(Other definitions for apostle that I've seen before include "Evangelist, proponent" , "To lapse (anag) - ardent supporter of a cause" , "Good man" , "Vigorous campaigner" , "One of Christ's 12 original disciples" .)