Missionary position? It's bitter outside! (7)
I believe the answer is:
apostle
'missionary' is the definition.
(I've seen this before)
'position? it's bitter outside' is the wordplay.
'position?' becomes 'post' (post is a kind of position).
'it's bitter' becomes 'ale' (type of beer or bitter).
'outside' is an insertion indicator (some letters go outside others).
'post' placed inside 'ale' is 'APOSTLE'.
(Other definitions for apostle that I've seen before include "Leaps to find teacher of faith" , "John, perhaps" , "Divine messenger" , "Good man" , "To lapse (anag) - ardent supporter of a cause" .)