Injury is hers, in treatment (6)
I believe the answer is:
shiner
'injury' is the definition.
(shiner is a kind of injury)
'hers in treatment' is the wordplay.
'treatment' is an anagram indicator (the letters must be processed or treated).
'hers'+'in'='hersin'
'hersin' with letters rearranged gives 'SHINER'.
'is' is the link.
(Other definitions for shiner that I've seen before include "device that lightens one's work" , "Her sin (anag)" , "Mouse" , "injury to face?" , "Light" .)