Vocalist's last, and the rest love backing group (5)
I believe the answer is:
octet
'group' is the definition.
(group of eight musicians)
'vocalist's last and the rest love backing' is the wordplay.
'last' says to take the final letters.
'and the rest' becomes 'etc' (et cetera).
'love' becomes 'o' (love means zero in tennis).
'backing' says the letters should be written backwards.
The last letter of 'vocalist' is 't'.
't'+'etc'+'o'='tetco'
'tetco' in reverse letter order is 'OCTET'.
(Other definitions for octet that I've seen before include "Composition for eight musicians" , "Musical work for eight" , "Eightsome" , "Piece for eight players" , "Mendelssohn or Schubert piece" .)