I get it's a sound made in mirth (3)
I believe the answer is:
aha
'i get it's' is the definition.
(expression said when understanding something)
'a sound made in mirth' is the wordplay.
'sound made in mirth' becomes 'ha' (sound of laughter).
'a'+'ha'='AHA'
(Other definitions for aha that I've seen before include "Exclamation of discovery or triumph" , "Triumphant cry" , "Expression of surprise, satisfaction or triumph" , "Exclamation of triumph or surprise, a palindrome" , "Bingo" .)