Happy-go-lucky, calm, ready — I’ve changed! (5-3-4)
I believe the answer is:
devil-may-care
'happy-go-lucky' is the definition.
(synonyms)
'calm ready i've changed' is the wordplay.
'changed' is an anagram indicator.
'calm'+'ready'+'ive'='calmreadyive'
'calmreadyive' is an anagram of 'DEVIL-MAY-CARE'.
(Other definitions for devil-may-care that I've seen before include "Reckless, audacious" , "Cheerfully reckless" , "Reckless - mediaeval cry (anag)" , "Cavalier" , "Insouciant (attitude)" .)