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' (synonyms).
'in' indicates putting letters inside.
'central' says to take the centre.
The middle of 'wales' is 'ale'.
'post' going 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" .)