Facial tan treatment is extreme (9)
I believe the answer is:
fanatical
'extreme' is the definition.
(I've seen this before)
'facial tan treatment' is the wordplay.
'treatment' indicates anagramming the letters (letters treated or processed differently).
'facial'+'tan'='facialtan'
'facialtan' with letters rearranged gives 'FANATICAL'.
'is' is the link.
(Other definitions for fanatical that I've seen before include "Mad" , "Excessively zealous" , "Unreasonably zealous" , "Extreme" , "Over-zealous" .)