Old city has exceptional ability in the beginning to produce fertiliser (4)
I believe the answer is:
urea
'the beginning to produce fertiliser' is the definition.
I can't judge whether this defines the answer.
'old city has exceptional ability' is the wordplay.
'old city' becomes 'ur' (ancient city state).
'has' is a charade indicator (letters next to each other).
'exceptional ability' becomes 'ea' (this might be a standard abbreviation of which I'm unaware).
'ur'+'ea'='UREA'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for urea that I've seen before include "organic compound" , "Waste product of mammals" , "It's produced by piddling" , "Compound found in urine, synthesised as a fertiliser" , "Waste matter" .)