Comic greeting America, including North and South American cities (9)
I believe the answer is:
hilarious
'comic' is the definition.
(thesaurus)
'greeting america including north and south american cities' is the wordplay.
I cannot quite see how this works, but
'greeting' could be 'hi' (hi is a kind of greeting) and 'hi' is found in the answer.
'america' could be 'us' (United States) and 'us' is found within the answer.
'american' could be 'a' (common abbreviation - e.g. in organisation names) and 'a' is found in the answer.
This explanation may well be incorrect...
(Other definitions for hilarious that I've seen before include "Priceless!" , "Uriah's oil (anag.)" , "Extremely funny" , "Extremely amusing" , "A scream" .)