Dog a matchless sort of boxer (6)
I believe the answer is:
shadow
'dog' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for shadow that I've seen before include "Follow - dark patch" , "Reflected image" , "Sort of boxer - dog" , "Dark area from which light is blocked" , "Partial or complete darkness" .)