Chatterbox makes contrary agent cover cost (6)
I believe the answer is:
yapper
'chatterbox' is the definition.
(I've seen this before)
'contrary agent cover cost' is the wordplay.
'contrary' is a reversal indicator.
'agent' becomes 'rep' (short for representative).
'cover cost' becomes 'pay' (I can't justify this - if you can you should believe this answer much more).
'rep'+'pay'='reppay'
'reppay' reversed gives 'YAPPER'.
'makes' is the link.
Can you help me to learn more?
(Other definitions for yapper that I've seen before include "garrulous commentator?" , "Chatterbox" , "Prepay (anag) -- an excited puppy?" , "One who can't stop talking" , "Noisy dog" .)