"I get it!" (3)
I believe the answer is:
aha
'i get it' is the definition.
(expression said when understanding something)
This is all the clue.
(Other definitions for aha that I've seen before include "Exclamation of triumph or mockery, a palindrome" , "Cry of triumph" , "I'm surprised" , "I get it!" , "Palindromic exclamation of triumph or surprise" .)