Brats deny disturbing onlooker (9)
I believe the answer is:
bystander
'onlooker' is the definition.
(I've seen this in another clue)
'brats deny disturbing' is the wordplay.
'disturbing' indicates an anagram.
'brats'+'deny'='bratsdeny'
'bratsdeny' is an anagram of 'BYSTANDER'.
(Other definitions for bystander that I've seen before include "Watcher of occurrence" , "Onlooker (often innocent)" , "Non-partcipating spectator" , "Bad sentry may be an onlooker" , "Non-participant spectator" .)