Missionary position in central Wales (7)
I believe the answer is:
apostle
'missionary' is the definition.
(I've seen this before)
'position in central wales' is the wordplay.
'position' becomes 'post' (post is a kind of position).
'in' means one lot of letters goes inside another.
'central' indicates the central letters.
The centre of 'wales' is 'ale'.
'post' placed inside 'ale' is 'APOSTLE'.
(Other definitions for apostle that I've seen before include "Early Christian teacher" , "Peter, perhaps" , "To lapse (anag) - ardent supporter of a cause" , "Ale pots (anag)" , "Teacher of new faith or movement" .)