Settle up and go on (3)
I believe the answer is:
yap
'go on' is the definition.
(both can mean to talk at length)
'settle up' is the wordplay.
'settle' becomes 'pay' (paying is a kind of settling).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'pay' in reverse letter order is 'YAP'.
'and' acts as a link.
(Other definitions for yap that I've seen before include "Natter; small dog's bark" , "High-pitched bark" , "gossip -" , "Bark incessantly" , "Shrill bark; wage reversed" .)