Volcanic material rises after mounted man causes disturbance (8)
I believe the answer is:
upheaval
'disturbance' is the definition.
(upheaval is a kind of disturbance)
'volcanic material rises after mounted man' is the wordplay.
'volcanic material' becomes 'lava' (I've seen this before).
'rises' shows that the letters should be reversed in order.
'after' says to put letters next to each other.
'mounted' becomes 'up' (I've seen this in other clues).
'man' becomes 'he'.
'lava' in reverse letter order is 'aval'.
'up'+'he'='uphe'
'aval' put after 'uphe' is 'UPHEAVAL'.
'causes' acts as a link.
(Other definitions for upheaval that I've seen before include "Great disruptive change" , "Disturbance that brings about great disruption" , "Turbulence" , "Violent or sudden change or disruption" , "Violent or sudden disturbance" .)