Injury is second in her treatment (6)
I believe the answer is:
shiner
'injury' is the definition.
(shiner is a kind of injury)
'second in her treatment' is the wordplay.
'second' becomes 's' (common abbreviation**).
'treatment' indicates anagramming the letters.
'in'+'her'='inher'
'inher' is an anagram of 'hiner'.
's'+'hiner'='SHINER'
'is' acts as a link.
(Other definitions for shiner that I've seen before include "injury to face?" , "A black eye, colloquially" , "Star" , "Black eye (colloq.)" , "Slang for a black-eye" .)