Big rodent on the loose in Kentish Town (9)
I believe the answer is:
tonbridge
'kentish town' is the definition.
(Tonbridge is a market town in Kent)
'big rodent on the loose' is the wordplay.
'on the loose' indicates an anagram.
'big'+'rodent'='bigrodent'
'bigrodent' anagrammed gives 'TONBRIDGE'.
'in' is the link.
(Other definitions for tonbridge that I've seen before include "Town in Kent" , "Kentish Town" , "Public school" .)