Seen teens, terrible state (9)
I believe the answer is:
tennessee
'state' is the definition.
(I know that Tennessee is a state)
'seen teens terrible' is the wordplay.
'terrible' indicates an anagram.
'seen'+'teens'='seenteens'
'seenteens' is an anagram of 'TENNESSEE'.
(Other definitions for tennessee that I've seen before include "Seen teens in this US state" , "Name of US dramatist" , "Volunteer State" , "Memphis's state" , "State of south-east US" .)