Killer whale in major catastrophe (4)
I believe the answer is:
'killer whale' is the definition.
('orca' can be a synonym of 'killer whale')
'in major catastrophe' is the wordplay.
'in' says the answer is hidden in the clue.
'ORCA' is hidden within 'major catastrophe'.
(Other definitions for orca that I've seen before include "Large predator" , "Cor! a killer whale" , "Killer in deep" , "Marine creature" , "Massive killer" .)