Escaped gorilla in old capital (7)
I believe the answer is:
rangoon
'old capital' is the definition.
(former capital of Myanmar)
'escaped gorilla' is the wordplay.
'escaped' becomes 'ran' (ran away).
'gorilla' becomes 'go on' (both can mean an aggressive man).
'ran'+'goon'='RANGOON'
'in' acts as a link.
(Other definitions for rangoon that I've seen before include "Former Burmese capital" , "It's now Yangon in Myanmar" , "Asian capital" , "Former capital of Burma" , "Port and largest city of Burma" .)