Eruption is fun (5)
I believe the answer is:
blast
I believe this clue is a double definition.
'eruption' is the first definition.
(synonyms)
'is fun' is the second definition.
(Other definitions for blast that I've seen before include "Severely criticise" , "Effect of explosion" , "Great time: explosion" , "slate" , "Loud noise from a horn" .)