Positioned with prejudice … Boatman is excluded (5)
I believe the answer is:
based
'positioned' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for based that I've seen before include "Founded, grounded" , "Worthless" , "Stationed (in a place)" , "(Of aircraft) stationed" , "Established" .)