Collier finds a large rock (7)
I believe the answer is:
mineral
'rock' is the definition.
'collier finds a large' is the wordplay.
'collier' becomes 'miner' (I've seen this in another clue).
'finds' is a charade indicator (letters next to each other).
'large' becomes 'L' (eg in clothes sizes).
'miner'+'a'+'l'='MINERAL'
(Other definitions for mineral that I've seen before include "Such as 1 dn" , "Neither animal nor vegetable" , "Substance needed for health; ore" , "Substance that is (usually) inorganic" , "Gypsum or magnesia, say" .)