Monster corporation in US city (3,5)
I believe the answer is:
big apple
'us city' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'monster corporation' is the wordplay.
'monster' becomes 'big'.
'corporation' becomes 'apple' (I can't justify this - if you can you should give a lot more credence to this answer).
'big'+'apple'='BIG APPLE'
'in' is the link.
Can you help me to learn more?
(Other definitions for big apple that I've seen before include "New York City (informal)" , "New York's fruity nickname" , "NY City nickname" , "Slang for New York citry" , "Nickname for New York" .)