Mineral found in quantity, but unnamed (5)
I believe the answer is:
umber
'mineral' is the definition.
(umber is a kind of mineral)
'quantity but unnamed' is the wordplay.
I cannot really understand how this works, but
'quantity' could be 'mb' (Mb is an example) and 'mb' is found in the answer.
This explanation may well be incorrect...
'found in' is the link.
Can you help me to learn more?
(Other definitions for umber that I've seen before include "Usually dark brown mineral" , "Earthy colour" , "Yellowish-brown pigment" , "Deep brown" , "Natural pigment" .)