Fish product around box on river in Kentish Town (9)
I believe the answer is:
rochester
'kentish town' is the definition.
(Rochester is a town in Kent)
'fish product around box on river' is the wordplay.
'fish product' becomes 'roe' (fish eggs).
'around' is an insertion indicator.
'box' becomes 'chest' (chest is a kind of box).
'on' is a charade indicator (letters next to each other).
'river' becomes 'r' (abbreviation for river).
'roe' placed around 'chest' is 'rocheste'.
'rocheste'+'r'='ROCHESTER'
'in' is the link.
(Other definitions for rochester that I've seen before include "Kentish Town" , "Kent town" , "Kent cathedral city" , "One city" , "Madwoman's husband" .)