Regretful of being so merry without me (5)
I believe the answer is:
sorry
'regretful' is the definition.
(synonyms)
'so merry without me' is the wordplay.
'without' indicates named letters should be taken away.
'merry' with 'me' taken away is 'rry'.
'so'+'rry'='SORRY'
'of being' acts as a link.
(Other definitions for sorry that I've seen before include "pathetic!" , "Contrite" , "I didn't quite catch that?" , "Common apology" , "Regretful, apologetic" .)