Biased a dreadful trial with father (7)
I believe the answer is:
partial
'biased' is the definition.
('partial' can be similar in meaning to 'biased')
'a dreadful trial with father' is the wordplay.
'a dreadful' is an anagram indicator.
'with' indicates putting letters inside.
'father' becomes 'pa' (pa is a kind of father**).
'trial' is an anagram of 'rtial'.
'rtial' going around 'pa' is 'PARTIAL'.
(Other definitions for partial that I've seen before include "One-sided" , "Prejudiced - incomplete" , "Biased or unfair" , "Incomplete; biassed" , "Keen" .)