Very brief moment (5,6)
![Ross](/clue/static/ross-mug.min.png)
'very brief moment' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for split second that I've seen before include "Twinkling of an eye" , "Very little time" , "Very brief moment of time" , "An extremely short time" , "Instant" .)