Editor upset by telecom company's bill (4)
I believe the answer is:
debt
'bill' is the definition.
(thesaurus)
'editor upset by telecom company's' is the wordplay.
'editor' becomes 'ed'.
'upset' says the letters should be written backwards.
'by' says to put letters next to each other.
'telecom company' becomes 'BT' (British Telecom).
'ed' written backwards gives 'de'.
'de'+'bt'='DEBT'
(Other definitions for debt that I've seen before include "Something owed" , "Financial liability" , "outstanding" , "What's due" , "The state of owing something" .)