Make biased or distorted (4)
I believe the answer is:
skew
I believe this clue is a double definition.
'make biased' is the first definition.
(I've seen this before)
'distorted' is the second definition.
(I've seen this before)
(Other definitions for skew that I've seen before include "Colour" , "Oblique; distort" , "Slant, bias" , "At an oblique angle" , "Aslant, bias" .)