Not up to game in Kentish Town (9)
I believe the answer is:
Here is my best explanation:
'kentish town' is the definition.
(market town in kent)
'not up to game' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'to game' becomes 'bridge' (bridge is a kind of game).
'not' written backwards gives 'ton'.
'in' is the link.
(Other definitions for tonbridge that I've seen before include "Public school", "Kentish Town", "Town in Kent" and "Kentish town".)