Some wassailers, very musical (5)
I believe the answer is:
assai
'very musical' is the definition.
(musical term for 'very')
'some wassailers' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'ASSAI' is hidden within 'wassailers'.
(Other definitions for assai that I've seen before include "attack in concert, for the most part" , "Very (in music scores)" , "Palm tree" , "Produced offspring" , "Very (in musical score)" .)