Facetious female's backchat mostly rubbish (8)
I believe the answer is:
flippant
'facetious' is the definition.
(thesaurus)
'female's backchat mostly rubbish' is the wordplay.
'female's backchat' becomes 'flip' (I can't explain this - if you can you should believe this answer much more).
'mostly' means to remove the last letter (most but not all of the word).
'rubbish' becomes 'pants' (informal term meaning poor-quality).
'pants' with its final letter removed is 'pant'.
'flip'+'pant'='FLIPPANT'
Can you help me to learn more?
(Other definitions for flippant that I've seen before include "Not serious or respectful" , "Impudent" , "showing disrespectful attitude" , "Showing disrespectful levity" , "Being facetious over pint flap" .)