Eg caterpillar right in middle of volcanic discharge (5)
I believe the answer is:
larva
'eg caterpillar' is the definition.
(caterpillar is a kind of larva)
'right in middle of volcanic discharge' is the wordplay.
'right' becomes 'r' (common abbreviation).
'in middle of' means one lot of letters goes inside another.
'volcanic discharge' becomes 'lava' (I've seen this before).
'r' placed into 'lava' is 'LARVA'.
(Other definitions for larva that I've seen before include "Insect in the first post-egg stage of its life" , "Immature life form" , "Immature form of caterpillar" , "Grub, caterpillar stage" , "Woodworm" .)