"Merry" month (3)
I believe the answer is:
may
'merry month' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for may that I've seen before include "Fifth month" , "Hawthorn blossom" , "Girl's name; 10 ac (anag.)" , "The permissive month" , "UK PM" .)