Smoking whilst stationary

hi all, i was in my car the other night sat in traffic only for about five minutes when smoke started to come from the passenger front wheel arch the smell was just of burning, i turned the car off and the smoke disapaited away i started the car back up fine no probs and drove away this was 4days ago and since then there has been nothing. i have spoken to a mechanic and reckons its the clutch is slipping, my car has only done 30000 miles so i dont understand why it would be slipping, the car itself drives fine chaning gears are fine too..so im really stumped as to what it could have been and is it likely to happen again, if anyone knows or has experienced this please tell me what you know or think it might be, all help gratefully received...cheers...