Housing development in heart of Gwent, say (6)
I believe the answer is:
estate
'housing development' is the definition.
(I've seen this before)
'heart of gwent say' is the wordplay.
'heart of' indicates the central letters.
'say' becomes 'state' ('state' can be a synonym of 'say').
The middle of 'gwent' is 'e'.
'e'+'state'='ESTATE'
'in' is the link.
(Other definitions for estate that I've seen before include "Property left at death" , "Country property" , "The whole of one's possessionsat death" , "What you leave when you die" , "Plantation" .)