Finally, Harry gets even - it's affirmative (3)
I believe the answer is:
yes
'it's affirmative' is the definition.
(affirmative answer word)
'finally harry gets even' is the wordplay.
'finally' says to take the final letters.
'even' indicates alternate letters (even-numbered letters are used).
The final letter of 'harry' is 'y'.
The alternate letters of 'gets' are 'es'.
'y'+'es'='YES'
(Other definitions for yes that I've seen before include "Absolutely" , "One response to "will you marry me?"" , "Affirmative answer" , "Validating vote" , "Word of agreement" .)