Mineral not fake guy brought up (7)
I believe the answer is:
realgar
'mineral' is the definition.
(I've seen this before)
'not fake guy brought up' is the wordplay.
'not fake' becomes 'real'.
'guy' becomes 'rag' (both can mean to tease or mock).
'brought up' is a reversal indicator (down clues only).
'rag' back-to-front is 'gar'.
'real'+'gar'='REALGAR'
(Other definitions for realgar that I've seen before include "rare find in a hot mineral spring" , "Soft red mineral" , "pigment" .)