Eruption is fun (5)
I believe the answer is:
blast
I believe this is a double definition.
'eruption' is the first definition.
(I know that eruption can be written as blast)
'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" .)