They are extremely upset work contains error (4)
I believe the answer is:
typo
'error' is the definition.
(typographical error)
'they are extremely upset work' is the wordplay.
'are extremely' means to remove the middle letters (only the extremes of the word are used).
'upset' is a reversal indicator.
'work' becomes 'op' (abbreviation for opus).
'they' with its middle taken out is 'ty'.
'op' in reverse letter order is 'po'.
'ty'+'po'='TYPO'
'contains' acts as a link.
(Other definitions for typo that I've seen before include "Keyboard slip-up" , "Misprint" , "Printer's error" , "Printer's mistake" , "Literal error in printed matter, in short" .)