Hard teen upset follower (8)
I believe the answer is:
adherent
'follower' is the definition.
(I know that adherent is a type of follower)
'hard teen upset' is the wordplay.
'upset' indicates an anagram.
'hard'+'teen'='hardteen'
'hardteen' with letters rearranged gives 'ADHERENT'.
(Other definitions for adherent that I've seen before include "Staunch follower" , "One supports" , "Supporter, follower" , "Loyal follower - sticks close" , "One supporting" .)