Hi,
"dern" is the Thai word for walking, so it means the fighter who is going forward. Sometimes the Thai commentator will be saying "Sylvie dern" and it means I'm the one coming forward, pressing the action.
Clinching is allowed to go for longer out in the provinces, medium in the stadia of Bangkok, and broken stupid quickly in new 3-round formats like Max, Thai Fight, Superchamp, Hardcore, MX, etc. They claim it's to make the fights more exciting, but it makes them more boring. It only gives people time to hold, rather than work. Golden Era fights like Samson Isaan vs. Pepsi (the third one especially) or Langsuan vs Lamnamoon, the clinch goes forever and the ref just kind of keeps them off the ropes. Breaking the clinch too fast would be like separating fighters after two strikes... totally interrupts the flow of the fight. But they claim it's to make it more exciting, but really it just makes it more like a Windmill show.
The way you see going forward as being positive, Thais see going backwards as being in control of the fight, rather than looking like you're scared. You can't ONLY go back. Dieselnoi often complains that all my opponents did was "nee" which is the Thai word for escaping. Going backwards for the sake of going backwards is as bad as going forward simply for the sake of going forward, it has to have meaning to how the fight is being directed by your movements. But a fighter who has the lead and then goes backwards to force the opponent to chase, or catch up, shows control of the fight. A fighter who is simply coming forward all the time looks desperate to a Thai eye, especially if they already have the lead, they look like they don't know that they're in the lead.