Mistake concealed by building site bully (9)
I believe the answer is:
terrorise
'bully' is the definition.
(I've seen this before)
'mistake concealed by building site' is the wordplay.
'mistake' becomes 'error' (synonyms).
'concealed by' means one lot of letters goes inside another.
'building' indicates anagramming the letters.
'site' anagrammed gives 'tise'.
'error' placed into 'tise' is 'TERRORISE'.
(Other definitions for terrorise that I've seen before include "Frighten or menace like roisterer" , "Oppress by fear" , "Frighten greatly" , "generate fear" , "Browbeat, bully" .)