Irregularity (7)
I believe the answer is:
anomaly
'irregularity' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for anomaly that I've seen before include "Special case" , "Deviation from the norm or expected" , "Irregularity or inconsistency, deviation from norm" , "eccentricity" , "irregular feature" .)