Not a big band (4)
I believe the answer is:
trio
'not a big band' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for trio that I've seen before include "Musical group" , "Bee Gees, Supremes or Andrews Sisters, say" , "Threesome" , "Beethoven's 'Archduke', for instance" , "wise men?" .)