A mouthful eaten by Swiss hero before a dance (10)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

An envelope (‘eaten by’) of ‘a’ plus RANT (‘mouthful’; Chambers gives for mouthful “an outburst of strong language” which is near enough) in TELL (William, him of the overture, ‘Swiss hero’) plus ‘a’.