Talk incessantly (5)
'talk incessantly' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for yak that I've seen before include "Himalayan ox" , "Hairy Tibetan ox" , "Persistent talk" , "Long-haired ox of SE Asia" , "Tibetan animal" .)
(Other definitions for gas that I've seen before include "Chatter (colloq.)" , "Neon, for example" , "Piped fuel" , "Empty talk (colloq.)" , "may fill American car" .)
'talk incessantly' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for yap that I've seen before include "Natter; small dog's bark" , "Complain shrilly" , "Shrill bark; rhymes with chart" , "Bark incessantly" , "Bark in quick sharp bursts" .)
(Another definition for runon that I've seen is " not using paragraphs?".)