Old rotten tree in West Country city (6)
I believe the answer is:
exeter
'west country city' is the definition.
(I've seen this before)
'old rotten tree in' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'rotten' is an anagram indicator.
'in' is an insertion indicator.
'tree' anagrammed gives 'eter'.
'ex' put into 'eter' is 'EXETER'.
(Other definitions for exeter that I've seen before include "Devon university city" , "South-west English city" , "City at S. end of M five" , "Cathedral city of southwest England" , "See" .)