Movement in E flat’s merry (6)
I believe the answer is:
festal
'merry' is the definition.
(thesaurus)
'movement in e flat's' is the wordplay.
'movement in' indicates anagramming the letters (I've seen this in other clues).
'e'+'flats'='eflats'
'eflats' is an anagram of 'FESTAL'.
(Other definitions for festal that I've seen before include "Joyous, festive" , "Jovial" , "Relating to a festival; joyous" , "Jovially celebratory" , "Relating to a holiday" .)