Money added to bill causes stress (6)
I believe the answer is:
accent
'stress' is the definition.
(synonyms)
'money added to bill' is the wordplay.
'money' becomes 'cent' (eg denomination of dollars).
'added to' says to put letters next to each other.
'bill' becomes 'ac' (short for account).
'cent' put after 'ac' is 'ACCENT'.
'causes' is the link.
(Other definitions for accent that I've seen before include "Particular emphasis" , "Emphasis on syllabus or word" , "feature of dialect?" , "Possibly a grave sign" , "intensity" .)