"Vaulting ambition, which o'erleaps _its sell_!"
_Sell_ is saddle (Latin, _sella_; French, _selle_), and
is used by Spenser in this sense.
"O'erleaping _itself_" is manifest nonsense; whereas the whole passage
has evident reference to horsemanship; and to "vault" is "to carry one's
body cleverly over anything of a considerable height, resting one hand
upon the thing itself,"--exactly the manner in which some persons mount
a horse, resting one hand on the pommel of the saddle.