It's a sign of stress, in a manner of speaking, for son leaving Constance (6)
I believe the answer is:
accent
'it's a sign' is the definition.
(accent is a kind of sign)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for accent that I've seen before include "Regional speech characteristic" , "Stress - manner of speech" , "Possibly a grave sign" , "feature of dialect?" , "It could be acute" .)