Indeed so horribly unfair (3-5)
I believe the answer is:
one-sided
'unfair' is the definition.
(I've seen this before)
'indeed so horribly' is the wordplay.
'horribly' is an anagram indicator (a horribly wrong version of the letters).
'indeed'+'so'='indeedso'
'indeedso' with letters rearranged gives 'ONE-SIDED'.
(Other definitions for one-sided that I've seen before include "Biased, favouring one part over another" , "Unfairly biased" , "Indeed so (anag.)" , "Unfair, unequal" , "Unfair or partisan - single dimension?" .)