Lot of material going off with TNT? It would cause explosions (10)
I believe the answer is:
antimatter
'it would cause explosions' is the definition.
(antimatter could cause explosions)
'lot of material going off with tnt?' is the wordplay.
'lot of' means to remove the last letter (I've seen 'lots of' mean this (the majority of the word)).
'going off with' is an anagram indicator.
'material' with its last letter removed is 'materia'.
'materia' mixed up with 'tnt' gives 'ANTIMATTER'.
(Other definitions for antimatter that I've seen before include "stuff with positrons" , "counterpart to our universe" , "it's potentially explosive" , "Counterpart to everyday substance" .)