Fine diamond, possibly, in English hair oil (12)
I believe the answer is:
brilliantine
'hair oil' is the definition.
(brilliantine is a kind of hair oil)
'fine diamond possibly in english' is the wordplay.
'fine diamond possibly' becomes 'brilliant' (brilliant can mean a very bright diamond).
'english' becomes 'E' (abbreviation).
'brilliant'+'in'+'e'='BRILLIANTINE'
(Other definitions for brilliantine that I've seen before include "Hairdresser" , "this smooths the waves!" , "preparation?" , "Scented hair oil" , "oil put on hair" .)