Minor deviation (4)
I believe the answer is:
blip
'minor deviation' 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 (and temporary) deviation from trend" , "Radar image; minor deviation" , "Echo displayed on a radar screen" , "shows up on radar screen" , "Unforeseen irregularity" .)