Either sing together or refrain (6)
I believe the answer is:
chorus
'refrain' is the definition.
(I know that refrain can be written as chorus)
'either sing together' is the wordplay.
I cannot quite see how this works, but
'either' could be 'or' (I've seen this in another clue) and 'or' is found within the answer.
This explanation may well be incorrect...
'or' is the link.
Can you help me to learn more?
(Other definitions for chorus that I've seen before include "Singing group - refrain" , "Dramatic team" , "Band of singers and dancers" , "Pipe up in unison" , "Group of singers as in musicals" .)