Service provided in mountainous area (6)
I believe the answer is:
massif
'mountainous area' is the definition.
(I've seen this before)
'service provided' is the wordplay.
'service' becomes 'mass' (church service).
'provided' becomes 'if' (eg 'provided it's sunny' means 'if it's sunny').
'mass'+'if'='MASSIF'
'in' is the link.
(Other definitions for massif that I've seen before include "series of peaks" , "Highland area?" , "Compact mountainous area" , "huge piece of crust" , "Compact grouping of mountains" .)