Fabulous colour, but get the nail polish remover (7)
I believe the answer is:
acetone
'the nail polish remover' is the definition.
(I've seen this in another clue)
'fabulous colour' is the wordplay.
'fabulous' becomes 'ace' (both can mean excellent).
'colour' becomes 'tone' (tone can mean a colour or shade).
'ace'+'tone'='ACETONE'
'but get' acts as a link.
(Other definitions for acetone that I've seen before include "Common solvent" , "Eat once (anag) - solvent" , "Inflammable liquid" , "Remover of nail polish" , "chemical is the solution" .)