Radar image showing temporary deviation (4)
I believe the answer is:
blip
I believe this clue is a double definition.
'radar image' is the first definition.
(I've seen this before)
'temporary deviation' is the second definition.
(I've seen this before)
'showing' is the link.
(Other definitions for blip that I've seen before include "Temporary setback - short high-pitched sound" , "Momentary deviation" , "Short-lived deviation" , "Temporary irregularity" , "Hiccup" .)