Monster source of goodness in mineral (4)
I believe the answer is:
ogre
'monster' is the definition.
(fairy tale monster)
'source of goodness in mineral' is the wordplay.
'source of' suggests taking the first letters.
'in' means one lot of letters goes inside another.
'mineral' becomes 'ore' (ore is a kind of mineral).
The initial letter of 'goodness' is 'g'.
'g' put into 'ore' is 'OGRE'.
(Other definitions for ogre that I've seen before include "man-eater" , "Has a monstrous role" , "Giant of folklore who liked to eat humans" , "Man-eating monster of fairy tales" , "Evil giant" .)