Chap writes off to the auditors (3)
I believe the answer is:
rex
'chap' is the definition.
'writes off to the auditors' is the wordplay.
'writes off' becomes 'wrecks' (to wreck a car can mean to write it off).
'to the auditors' indicates a 'sounds like' (homophone) clue (auditor can mean a person who listens).
'wrecks' is a homophone of 'REX'.
(Other definitions for rex that I've seen before include "King's title" , "Dog's name; king" , "Part of official title of a king" , "King (L.)" , "King - Harrison?" .)