Blast a variety of rock (6)
I believe the answer is:
basalt
'rock' is the definition.
(basalt is a kind of rock)
'blast a variety' is the wordplay.
'variety' indicates anagramming the letters.
'blast'+'a'='blasta'
'blasta' anagrammed gives 'BASALT'.
'of' acts as a link.
(Other definitions for basalt that I've seen before include "Common volcanic rock" , "Type of rock is a blast, oddly" , "Black volcanic rock" , "Commonest type of solidified lava; dense, dark grey and fine-grained" , "Dark rock of volcanic origin" .)