Deviation from the norm or expected (7)
I believe the answer is:
anomaly
'deviation from the norm or expected' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for anomaly that I've seen before include "Irregularity or inconsistency, deviation from norm" , "Deviation from the norm or common" , "Unexpected result" , "There's curiosity" , "irregular feature" .)