Twice harbouring a prejudice (4)
I believe the answer is:
bias
'prejudice' is the definition.
(I know that prejudice can be written as bias)
'twice harbouring a' is the wordplay.
'twice' becomes 'bis' (I've seen this in another clue).
'harbouring' indicates putting letters inside.
'bis' going around 'a' is 'BIAS'.
(Other definitions for bias that I've seen before include "A leaning or prejudice" , "Prejudice, one-sidedness" , "Inclination of prejudice" , "An influence against, prejudice" , "One-sided prejudice" .)