Volcano peak once exploded (4)
I believe the answer is:
cone
'volcano peak' is the definition.
(I've seen this before)
'once exploded' is the wordplay.
'exploded' is an anagram indicator.
'once' is an anagram of 'CONE'.
(Other definitions for cone that I've seen before include "Road furniture!" , "often full of ice" , "Tapered object" , "Road works marker" , "Element of geometry" .)