City can produce old subject (5)
I believe the answer is:
latin
'old subject' is the definition.
'latin' can be an answer for 'subject' (I have seen 'difficult subject' mean 'latin' so perhaps 'subject' could also mean 'latin'). I'm unsure of the 'old' bit.
'city can' is the wordplay.
'city' becomes 'LA' (Los Angeles).
'can' becomes 'tin' (both are metal containers).
'la'+'tin'='LATIN'
'produce' acts as a link.
Can you help me to learn more?
(Other definitions for latin that I've seen before include "Cicero's tongue" , "Language of Cicero" , "Shakespeare's was small, according to Jonson" , "Parent language of French,Romanian" , "Old Roman language" .)