The talk of many a mad person (7)
I believe the answer is:
chatter
'the talk' is the definition.
(chatter is a kind of talk)
'many a mad person' is the wordplay.
'many' becomes 'c' (C is the Roman numeral for 100).
'a mad person' becomes 'hatter' (ref to Lewis Carroll's mad hatter character).
'c'+'hatter'='CHATTER'
'of' acts as a link.
(Other definitions for chatter that I've seen before include "Prattle" , "Noisy talk" , "Babble or gossip" , "Idle gossip" , "Gas" .)