A black eye can be illuminating, it seems! (6)
I believe the answer is:
shiner
'a black eye' is the definition.
(I know that black eye can be written as shiner)
'be illuminating it seems' is the wordplay.
I cannot really understand how this works, but
'be' could be 'shine' (shining is a kind of being) and 'shine' is found within the answer.
A single letter 'r' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
'can' acts as a link.
Can you help me to learn more?
(Other definitions for shiner that I've seen before include "injury to face?" , "device that lightens one's work" , "It may result from punch" , "A slangy black eye" , "Black eye (colloq.)" .)