Go on too long (7)
'go on too long' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for overrun that I've seen before include "Exceed (allowed time/cost)" , "Move too fast" , "Cover - attack" , "Infest - defeat totally" , "(Programme) take too long; defeat easily" .)
(Other definitions for yak that I've seen before include "Long-haired ox of central Asia" , "Himalayan ox" , "Gossip; ox" , "Gossip (sl)" , "Tibetan ox - chatter" .)