Talk and talk and talk (6)
(Other definitions for yammer that I've seen before include "lament" , "jaw" , "Talk loudly at length" , "Talk loudly and incessantly" , "Wail" .)
'talk and talk and talk' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for yap that I've seen before include "Small dog's sound" , "Gab" , "Bark incessantly" , "gossip -" , "Bark like a tiny dog - speak constantly" .)