Substance found in mine — hard (4)
I believe the answer is:
pith
'substance' is the definition.
('pith' can be a synonym of 'substance')
'mine hard' is the wordplay.
'mine' becomes 'pit' (down the pit).
'hard' becomes 'h'.
'pit'+'h'='PITH'
'found in' is the link.
(Other definitions for pith that I've seen before include "Fibrous part of fruit - essence" , "Citrus rind lining" , "Medulla" , "Marrow (of plant)" , "hats may be made of this" .)