Second stroke (4)
I believe the answer is:
tick
I believe this clue is a double definition.
'second' is the first definition.
(both can mean a very short amount of time)
'stroke' is the second definition.
(thesaurus)
(Other definitions for tick that I've seen before include "Flash - bloodsucker" , "Credit (slang)" , "Mark correct, parasite" , "Mark of work done (right)" , "Small parasite" .)