Missionary position in middle of Wales (7)
I believe the answer is:
apostle
'missionary' is the definition.
(thesaurus)
'position in middle of wales' is the wordplay.
'position' becomes 'post' (post is a kind of position**).
'in' is an insertion indicator.
'middle of' indicates the central letters.
The middle of 'wales' is 'ale'.
'post' going inside 'ale' is 'APOSTLE'.
(Other definitions for apostle that I've seen before include "Champion - one of twelve" , "'Matthew or Mark, say (7)'" , "Maybe Peter" , "James, perhaps" , "Religious messenger" .)