Yes, at long last is happy (3)
I believe the answer is:
gay
'happy' is the definition.
('gay' can be similar in meaning to 'happy')
'yes at long last' is the wordplay.
'yes' becomes 'ay'.
'at' is a charade indicator (letters next to each other) (I've seen this in other clues).
'last' indicates one should take the final letters.
The last letter of 'long' is 'g'.
'ay' after 'g' is 'GAY'.
'is' acts as a link.
(Other definitions for gay that I've seen before include "Light-hearted, carefree" , "John --, author of 'The Beggar's Opera'" , "Writer of The Beggar's Opera" , "Homosexual" , "Merry" .)