Sudden seizure? (4)
I believe the answer is:
'sudden seizure?' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for grab that I've seen before include "Catch hold (of)" , "'Seize, snatch (4)'" , "Grasp roughly" , "Seize roughly" , "Seize rudely" .)