Couples in scar unit call for bruise application (6)
I believe the answer is:
arnica
'application' is the definition.
(plant used as a medical application)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for arnica that I've seen before include "Tincture of mountain tobacco, good for bruises, etc" , "flowering plant" , "Tincture for treating bruises" , "medicinal preparation" , "Herb used for treating bruises" .)