Rock shattered by a blast (6)
I believe the answer is:
basalt
'rock' is the definition.
(basalt is a kind of rock)
'shattered by a blast' is the wordplay.
'shattered' indicates anagramming the letters.
'by' means one lot of letters go next to another.
'a' after 'blast' is 'blasta'.
'blasta' with letters rearranged gives 'BASALT'.
(Other definitions for basalt that I've seen before include "Rock containing olivine" , "Commonest type of solidified lava; dense, dark grey and fine-grained" , "rock from 2 [VOLCANO]" , "Type of rock is a blast, oddly" , "Black volcanic rock" .)