Unpleasant to look at (4)
I believe the answer is:
ugly
'unpleasant to look at' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ugly that I've seen before include "Hideous" , "Not nice to look at" , "Menacing" , "Not pleasing to look at" , "Lacking in beauty" .)