Be of service to one stuck in revolting volcanic discharge (5)
I believe the answer is:
avail
'be of service to' is the definition.
(I've seen this before)
'one stuck in revolting volcanic discharge' is the wordplay.
'one' becomes 'I' (Roman numeral).
'stuck in' indicates putting letters inside.
'revolting' says the letters should be written in reverse.
'volcanic discharge' becomes 'lava' (lava is released in a volcanic eruption).
'lava' back-to-front is 'aval'.
'i' going within 'aval' is 'AVAIL'.
(Other definitions for avail that I've seen before include "Be of help or advantage" , "Be useful" , "Make use, take advantage" , "Be of value to" , "Help, benefit" .)