Couple in Bassenthwaite married (4)
I believe the answer is:
item
'couple' is the definition.
(a romantic couple are 'an item')
'in bassenthwaite married' is the wordplay.
'in' indicates a hidden word.
'ITEM' is hidden within 'bassenthwaite married'.
(Other definitions for item that I've seen before include "Unit; detail" , "pair?" , "Separate object or unit" , "story" , "partners" .)