Cap on the old man in American city (5)
I believe the answer is:
tampa
'american city' is the definition.
(I've seen this before)
'cap on the old man' is the wordplay.
'cap' becomes 'tam' (tam is a kind of cap).
'on' means one lot of letters go next to another.
'the old man' becomes 'pa' (both can mean father).
'tam'+'pa'='TAMPA'
'in' acts as a link.
(Other definitions for tampa that I've seen before include "US resort" , "Port of western Florida" , "US city" , "stateside destination" , "City in Florida" .)