Volcanic material I dropped from use after reflection (4)
I believe the answer is:
lava
'volcanic material' is the definition.
(I've seen this before)
'i dropped from use after reflection' is the wordplay.
'dropped from' indicates named letters should be taken away.
'use' becomes 'avail' (availing is a kind of using).
'after reflection' says the letters should be written in reverse.
'avail' with 'i' taken out is 'aval'.
'aval' in reverse letter order is 'LAVA'.
(Other definitions for lava that I've seen before include "Molten volcanic rock" , "Volcanic stuff" , "Volcanic flowing substance" , "Volcanic outflow" , "Stuff heatedly hurled out" .)