Leave old capital (5)
I believe the answer is:
quito
'capital' is the definition.
(Quito is an example)
'leave old' is the wordplay.
'leave' becomes 'quit' (quitting is a kind of leaving**).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'quit'+'o'='QUITO'
(Other definitions for quito that I've seen before include "City" , "Ecuadorian capital" , "S America" , "Capital of Ecuador" .)