Be excellent with gloss (5)
I believe the answer is:
shine
'be' is the definition.
(shining is a kind of being)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for shine that I've seen before include "Lustre" , "Give off bright light" , "Beam" , "Glimmer" , "Radiate or excel" .)