Mineral on board? (5)
I believe the answer is:
emery
This clue could be a double definition.
'mineral' is the first definition.
(emery is a kind of mineral)
'board?' is the second definition.
The answer and definition can be both substances as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'on' is the link.
Can you help me to learn more?
(Other definitions for emery that I've seen before include "Rock used as abrasive" , "Abrasive material" , "Dick - - , late comedian" , "Hard, grey-black mineral used as 11 down" , "Abrasive powder" .)