Suggestive, nothing unknown (7)
I believe the answer is:
'suggestive' is the definition.
'nothing unknown' is the wordplay.
'nothing' becomes 'naught' (synonyms).
'unknown' becomes 'y' (algebra).
(Other definitions for naughty that I've seen before include "Badly behaved, child say" , "Mischievous - improper" , "Badly-behaved" , "Badly behaved, mischievous" , "Dubious" .)