Unjust if a run, perhaps (6)
I believe the answer is:
unfair
'unjust' is the definition.
(I know that unjust can be written as unfair)
'if a run perhaps' is the wordplay.
'perhaps' indicates anagramming the letters.
'if'+'a'+'run'='ifarun'
'ifarun' is an anagram of 'UNFAIR'.
(Other definitions for unfair that I've seen before include "Not just - lacking in beauty" , "Not equitable-or blonde?" , "raining perhaps?" , "deceitful" , "Unjust, partial" .)