Explanation given in Sheen (5)
I believe the answer is:
gloss
I believe this clue is a double definition.
'explanation' is the first definition.
(I know that gloss is a type of explanation)
'sheen' is the second definition.
(I've seen this before)
'given in' is the link.
(Other definitions for gloss that I've seen before include "phrase's explanation" , "Polish - explanatory comment" , "High shine" , "Shiny surface" , "translation" .)