Advert around carnival city showing butt (9)
I believe the answer is:
posterior
'butt' is the definition.
(synonyms)
'advert around carnival city' is the wordplay.
'advert' becomes 'poster' (I have seen 'Large advert ' mean 'poster' so perhaps 'advert' could also mean 'poster').
'around' means one lot of letters goes inside another.
'carnival city' becomes 'Rio' (Rio de Janeiro is famous for its carnival).
'poster' placed around 'rio' is 'POSTERIOR'.
'showing' is the link.
(Other definitions for posterior that I've seen before include "Rear" , "Backside" , "Further back; bottom" , "Stern" , "later" .)