Graduate seasoning rock (6)
I believe the answer is:
basalt
'rock' is the definition.
(I know that basalt is a type of volcanic rock)
'graduate seasoning' is the wordplay.
'graduate' becomes 'BA' (someone with a Bachelor of Arts degree).
'seasoning' becomes 'salt' (salt is a kind of seasoning).
'ba'+'salt'='BASALT'
(Other definitions for basalt that I've seen before include "Dark fine-grained volcanic rock" , "Type of rock is a blast, oddly" , "Commonest type of solidified lava; dense, dark grey and fine-grained" , "Common volcanic rock" , "rock from 2 [VOLCANO]" .)