Sudden outburst from colossal volcano. (5)
I believe the answer is:
salvo
'sudden outburst' is the definition.
(I've seen this in another clue)
'from colossal volcano' is the wordplay.
'from' indicates a hidden word.
'SALVO' is hidden in the letters of 'colossal volcano'.
(Other definitions for salvo that I've seen before include "Broadside" , "Burst of rapid fire" , "Simultaneous discharge of artillery" , "Succession of shots" , "Round of fire" .)