Minor anomaly (4)
I believe the answer is:
blip
'minor anomaly' 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 "Temporary irregularity" , "Minor and temporary problem" , "Short-lived deviation" , "Signal on a radar screen" , "Unforeseen irregularity" .)