Lout beginning to cause mayhem when officer leaves (4)
I believe the answer is:
chav
'lout' is the definition.
(derogatory term for a lower-class person)
I don't understand the rest of the clue.
(Other definitions for chav that I've seen before include "Denoting type of person stereotyped by bling and baseball caps" , "Boorish type" , "Young person given to loutish behaviour and designer clothes" , "Lout" , "Baseball-capped, badly dressed, ignorant youth" .)