At mount, about to be lacking lustre (4)
I believe the answer is:
matt
'lacking lustre' is the definition.
The answer and definition can be both human attributes as well as being singular nouns.
Maybe there's a link between them I don't understand?
'at mount about' is the wordplay.
'mount' becomes 'mt'.
'about' means one lot of letters goes inside another.
'at' inserted within 'mt' is 'MATT'.
'to be' acts as a link.
Can you help me to learn more?
(Other definitions for matt that I've seen before include "gospel!" , "Lacking a shine" , "- Lucas (Little Britain)" , "Dull uniform finish" , "book" .)