Bully makes mistake in site management (9)
I believe the answer is:
terrorise
'bully' is the definition.
('bully'->'terrorize' is in my internal thesaurus and 'ize'->'ise')
'mistake in site management' is the wordplay.
'mistake' becomes 'error' ('error' can be a synonym of 'mistake').
'in' is an insertion indicator.
'management' is an anagram indicator.
'site' anagrammed gives 'tise'.
'error' going within 'tise' is 'TERRORISE'.
'makes' is the link.
(Other definitions for terrorise that I've seen before include "Panic" , "generate fear" , "Frighten or menace like roisterer" , "Spook" , "Frighten greatly" .)