Stress when money is added to bill (6)
I believe the answer is:
accent
'stress' is the definition.
('accent' can be a synonym of 'stress')
'money is added to bill' is the wordplay.
'money' becomes 'cent' (I've seen this before).
'is added to' is a charade indicator (letters next to each other).
'bill' becomes 'ac'.
'cent' put after 'ac' is 'ACCENT'.
'when' is the link.
(Other definitions for accent that I've seen before include "Acute, perhaps" , "Grave, perhaps" , "Mark over a vowel, perhaps" , "Emphasis on syllable or local pronunciation" , "feature of dialect?" .)