Genuine fish pigment (7)
I believe the answer is:
realgar
'pigment' is the definition.
(thesaurus)
'genuine fish' is the wordplay.
'genuine' becomes 'real' ('real' is associated in meaning with 'genuine').
'fish' becomes 'gar' (gar is a kind of fish).
'real'+'gar'='REALGAR'
(Other definitions for realgar that I've seen before include "rare find in a hot mineral spring" , "pigment" , "Soft red mineral" .)