Radar screen image
I believe the answer is:
blip
'radar screen image' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for blip that I've seen before include "Signal on a radar screen" , "Momentary deviation" , "Unexpected and minor temporary deviation" , "Temporary setback - short high-pitched sound" , "Small mark - unforeseen deviation" .)