Replaces black gold -- substance found just below the surface (7)
I believe the answer is:
subsoil
'found just below the surface' is the definition.
'replaces black gold' is the wordplay.
'replaces' becomes 'subs' (as in to bring on a substitute in sport).
'black gold' becomes 'oil' (I've seen this in another clue).
'subs'+'oil'='SUBSOIL'
'substance' acts as a link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for subsoil that I've seen before include "Material between surface and bedrock" , "The lower earth, so to speak" , "that's just below the surface?" , "It's used to cultivate" , "just below the ground" .)