Enthusiasm to blow over at the outset (5)
'enthusiasm' is the definition.
(gusto is a kind of enthusiasm)
'blow over at the outset' is the wordplay.
'blow' becomes 'gust' (synonyms).
'at the outset' says to take the initial letters.
The initial letter of 'over' is 'o'.
'to' is the link.
(Other definitions for gusto that I've seen before include "Great relish", "Zest and enjoyment in doing something", "Great enjoyment in doing something", "Vigorous and enthusiastic enjoyment", "Obvious enthusiasm".)