Idle talk about a madman (7)
I believe the answer is:
chatter
'idle talk' is the definition.
(I've seen this before)
'about a madman' is the wordplay.
'about' becomes 'c' (abbreviation for 'circa').
'a madman' becomes 'hatter' (mad as a hatter).
'c'+'hatter'='CHATTER'
(Other definitions for chatter that I've seen before include "Talk rapidly" , "Talk idly" , "Talk noisily" , "Prattle" , "Rabbit on" .)