Finds it difficult to keep city clean (9)
I believe the answer is:
disinfect
'clean' is the definition.
(disinfecting is a kind of cleaning)
'finds it difficult to keep city' is the wordplay.
'difficult' indicates an anagram.
'to keep' means one lot of letters goes inside another.
'city' becomes 'ec' (the City of London postcode).
'finds'+'it'='findsit'
'findsit' is an anagram of 'disinft'.
'disinft' going around 'ec' is 'DISINFECT'.
(Other definitions for disinfect that I've seen before include "Clean so as to destroy bacteria" , "Make pure" , "get rid of bugs" , "purge" , "Sanitise" .)