Were it a clean fresh cut you could carefully superglue it (emergency wound closure is what superglue was originally developed for). However, although I can't get a clear view, it looks more like a tear, or burst abscess. You can use blue spray near the mouth - doesn't taste so good, but not harmful. I would probably also use a long acting antibiotic - because it looks dirty, so there is a definite possibility of infection particularly as it's round the mouth.
If you are unsure, ask your vet's advice.