Small combo (4)
I believe the answer is:
trio
'small combo' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for trio that I've seen before include "Work for three" , "Archduke, possibly" , "Beethoven's 'Archduke', for instance" , "Riot (anag)" , "A group or set of three" .)