Radar image (4)
I believe the answer is:
blip
'radar image' 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 "often seen on radar screen" , "Temporary lapse" , "Minor (and temporary) deviation from trend" , "Image of object on radar screen" , "Radar image; minor deviation" .)