Exclamation of triumph (3)
I believe the answer is:
aha
'exclamation of triumph' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for aha that I've seen before include "it expresses surprise" , "Palindromic exclamation of triumph or surprise" , "Cry of surprise" , "Cry of triumph" , "Bingo" .)