Talk on and on (4)
(Other definitions for ramble that I've seen before include "Walk or wander" , "Walk in the countryside" , "Walk; be discursive" , "Walk desultorily (or in a group)" , "Wander about aimlessly" .)
'talk on and on' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for yack that I've seen before include "Natter" , "Talk incessantly" , "Prattle" .)