Family member taken aback about a prejudice (4)
I believe the answer is:
bias
'prejudice' is the definition.
('bias' can be a synonym of 'prejudice')
'family member taken aback about a' is the wordplay.
'family member' becomes 'sib' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'taken aback' shows that the letters should be reversed in order.
'about' is an insertion indicator.
'sib' in reverse letter order is 'bis'.
'bis' placed around 'a' is 'BIAS'.
Can you help me to learn more?
(Other definitions for bias that I've seen before include "One-sided inclination" , "Unfairly influence" , "Prejudiced or weighted opinion" , "Prejudice for one side" , "Inclination of prejudice" .)