No ancient city is hard to feed (7)
I believe the answer is:
nourish
'feed' is the definition.
(I know that feed is a is a kind of of nourish)
'no ancient city is hard' is the wordplay.
'ancient city' becomes 'ur' (ancient Sumerian city state).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'no'+'ur'+'is'+'h'='NOURISH'
'to' acts as a link.
(Other definitions for nourish that I've seen before include "Provide with materials for life and growth" , "Provide with food for growth" , "support" , "Keep alive and well with food" , "Supply with food, keep well" .)