Many a mineral is comparatively costly (4)
I believe the answer is:
more
'many' is the definition.
(associated in meaning)
'a mineral is comparatively costly' is the wordplay.
I cannot really understand how this works, but
'mineral' could be 'ore' (ore is a kind of mineral) and 'ore' is found in the answer.
A single letter 'm' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for more that I've seen before include "author" , "victim of Henry" , "Extra (for Sir Thomas?)" , "Greater quantity or number" , "Additional or larger amount" .)