Sudden fit (8)
I believe the answer is:
paroxysm
'sudden fit' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for paroxysm that I've seen before include "Sudden uncontrollable outburst - fit" , "Sudden outburst, of coughing say" , "Fit - convulsion" , "Spasm" , "Uncontrollable fit (of giggles, coughing etc)" .)