Terribly cool artist joins party in a state (8)
I believe the answer is:
colorado
'in a state' is the definition.
(I've seen this before)
'terribly cool artist joins party' is the wordplay.
'terribly' indicates an anagram.
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'joins' means one lot of letters go next to another.
'party' becomes 'do' (do can informally mean a party).
'cool' is an anagram of 'colo'.
'colo'+'ra'+'do'='COLORADO'
(Other definitions for colorado that I've seen before include "US state has a strangely cool road" , "in America?" , "US state in the Rocky Mountains" , "Boulder's state" , "Denver's state" .)