Explosively blast a rock (6)
I believe the answer is:
basalt
'rock' is the definition.
(basalt is a kind of rock)
'explosively blast a' is the wordplay.
'explosively' is an anagram indicator (I've seen this in other clues).
'blast'+'a'='blasta'
'blasta' with letters rearranged gives 'BASALT'.
(Other definitions for basalt that I've seen before include "Black volcanic rock" , "Common volcanic rock" , "Type of rock is a blast, oddly" , "rock from 2 [VOLCANO]" , "Commonest type of solidified lava; dense, dark grey and fine-grained" .)