Overenthusiastic facial tan going wrong (9)
I believe the answer is:
fanatical
'overenthusiastic' is the definition.
(thesaurus)
'facial tan going wrong' is the wordplay.
'going wrong' indicates an anagram.
'facial'+'tan'='facialtan'
'facialtan' with letters rearranged gives 'FANATICAL'.
(Other definitions for fanatical that I've seen before include "Unreasonably zealous" , "Obsessive" , "Blindly enthusiastic" , "Overenthusiastic" , "Excessively zealous" .)