Discriminatory (6)
'discriminatory' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for unfair that I've seen before include "out of order" , "Affected by partiality - dishonest" , "Not equitable, biased" , "Not just < dark" , "'Unjust, inequitable (6)'" .)
(Another definition for prejudicial that I've seen is " Harmful, detrimental".)