Possibly blast a rock (6)
I believe the answer is:
basalt
'rock' is the definition.
(I know that basalt is a type of volcanic rock)
'possibly blast a' is the wordplay.
'possibly' is an anagram indicator.
'blast'+'a'='blasta'
'blasta' anagrammed gives 'BASALT'.
(Other definitions for basalt that I've seen before include "Dark rock of volcanic origin" , "Rock containing olivine" , "Commonest type of solidified lava; dense, dark grey and fine-grained" , "Black volcanic rock" , "rock from 2 [VOLCANO]" .)