Betting on old city stimulus (4)
I believe the answer is:
spur
'stimulus' is the definition.
(I've seen this before)
'betting on old city' is the wordplay.
'betting' becomes 'sp'.
'on' is a charade indicator (letters next to each other).
'old city' becomes 'ur' (ancient city state).
'sp'+'ur'='SPUR'
(Other definitions for spur that I've seen before include "Cowboy boot attachment" , "Goad, prod" , "Incentive; branch line" , "Projection on cock's leg" , "Spike on a horse-rider's boot" .)