A blast shattered the rock (6)
I believe the answer is:
basalt
'the rock' is the definition.
(basalt is a kind of rock)
'a blast shattered' is the wordplay.
'shattered' is an anagram indicator.
'a'+'blast'='ablast'
'ablast' with letters rearranged gives 'BASALT'.
(Other definitions for basalt that I've seen before include "Commonest type of solidified lava; dense, dark grey and fine-grained" , "Dark fine-grained volcanic rock" , "Rock containing olivine" , "rock from 2 [VOLCANO]" , "Common volcanic rock" .)