Slip off coat, put on a short frock and make merry (6)
I believe the answer is:
frolic
'merry' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for frolic that I've seen before include "Lark - gambol" , "Cut capers" , "Caper, cavort" , "Gambol, play merrily" , "Playful action or movement" .)