Group included in ban on Etonians (5)
I believe the answer is:
nonet
'group' is the definition.
(group of nine musicians)
'included in ban on etonians' is the wordplay.
'included in' indicates a hidden word.
'NONET' is hidden in the letters of 'ban on etonians'.
(Other definitions for nonet that I've seen before include "Composition for nine instruments" , "musical piece" , "Composition for nine performers" , "group composition" , "piece to be performed" .)