Blow one's initial enthusiasm (5)
I believe the answer is:
gusto
'enthusiasm' is the definition.
(gusto is a kind of enthusiasm)
'blow one's initial' is the wordplay.
'blow' becomes 'gust' (as in a gust of wind).
'initial' says to take the initial letters.
The first letter of 'ones' is 'o'.
'gust'+'o'='GUSTO'
(Other definitions for gusto that I've seen before include "Zest, zeal" , "ardour" , "Exuberant enjoyment" , "Great enjoyment, vigour" , "Got us with great enjoyment" .)