Ruler pirouetting in merriment (4)
I believe the answer is:
emir
'ruler' is the definition.
(historical term for a Muslim ruler)
'pirouetting in merriment' is the wordplay.
'pirouetting' is a reversal indicator (pirouette is a spin in ballet - letters spin round).
'in' says the answer is hidden in the clue.
'rime' is hidden within 'merriment'.
'rime' reversed gives 'EMIR'.
(Other definitions for emir that I've seen before include "Ruler in the Middle East and mire" , "Middle Eastern ruler" , "Eastern title" , "leader in the East" , "Title of various Muslim rulers" .)