Good book in head office beginning to cause mayhem (5)
I believe the answer is:
havoc
'mayhem' is the definition.
(I know that mayhem can be written as havoc)
'good book in head office beginning to cause' is the wordplay.
'good book' becomes 'AV' (Authorised Version of the Bible).
'in' means one lot of letters goes inside another.
'head office' becomes 'HO'.
'beginning to' indicates taking the first letters.
The initial letter of 'cause' is 'c'.
'av' inserted within 'ho' is 'havo'.
'havo'+'c'='HAVOC'
(Other definitions for havoc that I've seen before include "Chaos and destruction" , "Carnage" , "Disaster" , "Widespread destruction and great disorder" , "Widepsread destruction" .)