Black sauce is something unusual (4)
I believe the answer is:
blip
'something unusual' is the definition.
(I know this)
'black sauce' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'sauce' becomes 'lip' (both can mean insolence).
'b'+'lip'='BLIP'
'is' is the link.
(Other definitions for blip that I've seen before include "Temporary lapse" , "Unexpected and minor temporary deviation" , "shows up on radar screen" , "Temporary irregularity" , "Momentary deviation" .)