Mineral brought down, said Pole (8)
I believe the answer is:
feldspar
'mineral' is the definition.
(feldspar is a kind of mineral)
'brought down said pole' is the wordplay.
I cannot really see how this works, but
'down' could be 'd' and 'd' is present in the answer.
'pole' could be 'spar' (spar is a kind of pole) and 'spar' is found in the answer.
This explanation may well be incorrect...
(Other definitions for feldspar that I've seen before include "Abundant mineral" , "Common mineral" , "crystal" , "Mineral; red flaps (anag.)" , "Rock-forming mineral" .)