Some backing from the choir trebles for a small group (4)
I believe the answer is:
trio
'a small group' is the definition.
(I've seen this before)
'some backing from the choir trebles' is the wordplay.
'some backing' says the letters should be written backwards.
'from' indicates a hidden word.
'oirt' can be found hidden inside 'choir trebles'.
'oirt' written backwards gives 'TRIO'.
'for' acts as a link.
(Other definitions for trio that I've seen before include "Archduke, possibly" , "Musical group" , "Bee Gees, Supremes or Andrews Sisters, say" , "Group of three (musicians)" , "Piece for three players" .)