i recently purchased a new quad.  On the trail the clutch suddenly would not engage, however on more level ground I was able to get a little grab (enough to make it back and toast whatever was left).  There was no sign a slippage or burning prior.  While I am not an expert rider, I am familiar with the techniques of riding sport machines and have been riding for many years now.  The clutch adjustment on the cable appeared to be fine prior.  Again this is a brand new machine and was not abused its first time out.  The dealer will not warranty this repair due to the smoked clutch.  My question is:  What could cause the brand new clutch to suddenly fail on the trail in 3 hours?     

Hate to hear that on a brand new machine . I traded in my horses for one and though i dont know a whole lot about them i do know this, i bought a brand new 350 raptor from a dealer and well i get to hear later on how crappy these people are. They did no pre lube or anything to the quad i bought. Things like  checking cables lubricating things checking torque on bolts. I rode this quad for 15 mins until i discovered all this , it was leaking oil out the head excessively,the clutch was not adjusted properly nor any of the brakes. I payed 300 dollars extra for all this pre lube crap and they didnt do it . But i found a good dealer that made things right! In that 15 mins i wore the clutch over 40 % out and all because of the dealer not taking time to be thorough anyways that may have been the reason yours did this and you can get Yamaha to fix it with alot of choice words and telling them your bringing it back they kinda hop to it !  

Do you know what kind of oil was used (synthetic or petroleum based or a Slick 50 type additive)?  Wet clutches should only use petroleum based oil and no Teflon based additives.  Non-petroleum based oils and additives cause the clutch to slip and wear out prematurely.