Replaces fossil fuel that is found just below the surface (7)
I believe the answer is:
subsoil
'is found just below the surface' is the definition.
'replaces fossil fuel' is the wordplay.
'replaces' becomes 'subs' (as in to bring on a substitute in sport).
'fossil fuel' becomes 'oil' (I've seen this before).
'subs'+'oil'='SUBSOIL'
'that' is the link.
(Other definitions for subsoil that I've seen before include "Material between surface and bedrock" , "may be dug up down under" , "It's used to cultivate" , "that's just below the surface?" , "just below the ground" .)