Rex is upset about money order in accounts department (9)
I believe the answer is:
exchequer
'accounts department' is the definition.
'rex is upset about money order' is the wordplay.
'is upset' is an anagram indicator.
'about' indicates putting letters inside.
'money order' becomes 'cheque' (I've seen this before).
'rex' anagrammed gives 'exr'.
'exr' placed around 'cheque' is 'EXCHEQUER'.
'in' acts as a link.
(Other definitions for exchequer that I've seen before include "Government funds" , "Purse" , "old government office" , "finance department" , "large supply of money" .)