It's impertinent, but piquant (5)
I believe the answer is:
sauce
'impertinent but piquant' is the definition.
I can't tell whether this definition defines the answer.
'it's' is the wordplay.
I cannot really see how this works, but
'it' could be 'sa' (old-fashioned abbreviation for sex appeal) and 'sa' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sauce that I've seen before include "'Hunger is the best . . . . .'" , "Gravy" , "Insolence" , "Relish - impudence" , "Dressing served with food" .)