Yes, by midnight is merry (3)
I believe the answer is:
gay
'merry' is the definition.
(synonyms)
'yes by midnight' is the wordplay.
'yes' becomes 'ay'.
'by' says to put letters next to each other.
'midnight' becomes 'g' (the middle letter in 'night').
'ay' put after 'g' is 'GAY'.
'is' is the link.
(Other definitions for gay that I've seen before include "John --, author of 'The Beggar's Opera'" , "Homosexual" , "Brightly coloured" , "Writer of The Beggar's Opera" , "Light-hearted, carefree" .)