Present opened merrily on December evening - really nice! (6)
I believe the answer is:
modern
'present' is the definition.
(thesaurus)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for modern that I've seen before include "Present-day" , "See 11" , "Recent, up to date" , "Activity of 18 ac" , "'Contemporary, up-to-the-minute (6)'" .)