Blink of an eye (4)
I believe the answer is:
wink
'blink of an eye' is the definition.
(synonyms)
This is all the clue.
(Other definitions for wink that I've seen before include "It may be tipped" , "Sleep moment; twinkle" , "... associated eye movement" , "Convey an amused understanding" , "Conspiratorial eye movement" .)