Mixture of aluminium in molten rock rejected (7)
I believe the answer is:
amalgam
'mixture' is the definition.
(amalgam is a kind of mixture)
'aluminium in molten rock rejected' is the wordplay.
'aluminium' becomes 'al' (chemical symbol).
'in' indicates putting letters inside.
'molten rock' becomes 'magma' (molten rock from a volcano).
'rejected' shows that the letters should be reversed in order.
'magma' in reverse letter order is 'amgam'.
'al' inserted inside 'amgam' is 'AMALGAM'.
'of' is the link.
(Other definitions for amalgam that I've seen before include "mercurial combination" , "often filling" , "Blend of metals" , "Mixture, blend" , "combination of things" .)