Expecting niece, ten, to be naughty (8)
I believe the answer is:
enceinte
'expecting' is the definition.
(both can mean pregnant)
'niece ten to be naughty' is the wordplay.
'to be naughty' is an anagram indicator.
'niece'+'ten'='nieceten'
'nieceten' is an anagram of 'ENCEINTE'.
(Other definitions for enceinte that I've seen before include "Like a woman expecting" , "enclosure" , "some fortification" .)