Cancun is only holding this concert (6)
I believe the answer is:
unison
'this concert' is the definition.
(to work in concert is to work in unison)
'cancun is only holding' is the wordplay.
'holding' says the answer is hidden in the clue.
'UNISON' is hidden within 'cancun is only'.
(Other definitions for unison that I've seen before include "Concordance" , "Complete agreement, harmony" , "All acting the same way at the same time" , "Concord; with same pitch" , "In agreement - like sound of choir" .)