Radar screen spot (4)
I believe the answer is:
blip
'radar screen spot' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for blip that I've seen before include "minor inconvenience" , "Hiccup" , "Momentary deviation" , "Radar image; minor deviation" , "Unexpected and minor temporary deviation" .)