Barrel through rolling volcanic area together (2,4)
I believe the answer is:
en bloc
'together' is the definition.
(I've seen this before)
'barrel through rolling volcanic area' is the wordplay.
'barrel' becomes 'bl' (abbreviation).
'through' is an insertion indicator (inserted letters go through the word).
'rolling' says the letters should be written in reverse.
'volcanic area' becomes 'cone' (I can't justify this - if you can you should give a lot more credence to this answer).
'cone' back-to-front is 'enoc'.
'bl' put within 'enoc' is 'EN BLOC'.
Can you help me to learn more?
(Other definitions for en bloc that I've seen before include "As a group (French)" , "As a group (Fr.)" , "All at the same time - in France" , "All together - as a whole" , "All together, en masse" .)