Chaff that's no good when old? (8)
I believe the answer is:
badinage
'chaff' is the definition.
(I've seen this before)
'no good when old?' is the wordplay.
'no good' becomes 'bad' (similar in meaning).
'when old?' becomes 'inage' (I can't explain this - if you can you should give a lot more credence to this answer).
'bad'+'inage'='BADINAGE'
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for badinage that I've seen before include "Wordplay" , "Humorous banter" , "A big Dane given to banter" , "bit of playful repartee" , "Humorous, witty conversation" .)