International group is performing in concert (6)
I believe the answer is:
unison
'concert' is the definition.
(to work in concert is to work in unison)
'international group is performing' is the wordplay.
I cannot quite understand how this works, but
'international' could be 'un' (I've seen this in other clues) and 'un' is present in the answer.
'is' is within the answer.
'performing' could be 'on' (as in being on stage) and 'on' is found within the answer.
No letters remain.
This explanation may well be incorrect...
'in' acts as a link.
Can you help me to learn more?
(Other definitions for unison that I've seen before include "Sameness in pitch" , "with regard to the pitch" , "Simultaneous utterance" , "Togetherness - TUC member" , "All acting the same way at the same time" .)