Tenor and the rest start in Opera backing group (5)
I believe the answer is:
octet
'group' is the definition.
(group of eight musicians)
'tenor and the rest start in opera backing' is the wordplay.
'tenor' becomes 't'.
'and the rest' becomes 'etc' (et cetera).
'start in' indicates taking the first letters.
'backing' is a reversal indicator.
The initial letter of 'opera' is 'o'.
't'+'etc'+'o'='tetco'
'tetco' reversed gives 'OCTET'.
(Other definitions for octet that I've seen before include "Set of eight" , "A group of eight, may be singers" , "Chamber ensemble, a Mendelssohn favourite" , "Medieval Latin hymn" , "Mendelssohn wrote this" .)