The ultimate in cataclysmal astonishment! (4)
I believe the answer is:
last
'the ultimate' is the definition.
(I've seen this before)
'in cataclysmal astonishment' is the wordplay.
'in' indicates the answer is hidden within the clue.
'LAST' is hidden within 'cataclysmal astonishment'.
(Other definitions for last that I've seen before include "most recent model" , "Cargo to stay fresh" , "The cobbler's equipment will endure" , "Bringing up the rear" , "model of repair" .)