Mark overheard plots (6)
I believe the answer is:
bruise
'mark' is the definition.
(mark left by an injury)
'overheard plots' is the wordplay.
'overheard' indicates a 'sounds like' (homophone) clue.
'plots' becomes 'brews' (brew can mean to plot or devise something).
'brews' is a homophone of 'BRUISE'.
(Other definitions for bruise that I've seen before include "Discolouration left by a blow" , "Mark left by blow" , "Contusion" , "Injure underlying soft tissue or bone" , "Superficial injury" .)