Rush after a cat - it could cause immense damage (4,4)
I believe the answer is:
'it could cause immense damage' is the definition.
'rush after a cat' is the wordplay.
'rush' becomes 'bomb'.
'after' says to put letters next to each other.
'cat' becomes 'tom' (a tom is a male cat).
'bomb' put after 'atom' is 'ATOM-BOMB'.
(Other definitions for atom bomb that I've seen before include "Big weapon" , "The first one was dropped on Hiroshima" , "It would deter" , "can be a big killer" , "Explosive causing huge cloud" .)