An opening in flat in Northamptonshire town (8)
I believe the answer is:
daventry
'northamptonshire town' is the definition.
'an opening in flat' is the wordplay.
'an' becomes 'a'.
'opening' becomes 'vent' (vent is a kind of opening).
'in' indicates putting letters inside.
'flat' becomes 'dry'.
'a'+'vent'='avent'
'avent' going inside 'dry' is 'DAVENTRY'.
'in' is the link.
(Other definitions for daventry that I've seen before include "town" , "place in Northants" .)