Canals spoiled their oddly northern city (9)
I believe the answer is:
lancaster
'northern city' is the definition.
(I've seen this before)
'canals spoiled their oddly' is the wordplay.
'spoiled' indicates an anagram.
'oddly' indicates alternate letters.
The alternating letters of 'their' are 'ter'.
'canals' is an anagram of 'lancas'.
'lancas'+'ter'='LANCASTER'
(Other definitions for lancaster that I've seen before include "WW2 bomber; city" , "15th Century house" , "A rose-red city" , "old house" , "World War Two bomber" .)