Similarity — portrait (8)
I believe the answer is:
likeness
This clue is a double definition.
'similarity' is the first definition.
(I know that likeness is a type of similarity)
'portrait' is the second definition.
(I've seen this in another clue)
(Other definitions for likeness that I've seen before include "Similarity in appearance" , "Similarity - faithful portrait" , "impression" , "Good portrait" , "Image - comparison" .)