Middle age comes with depletion of lustre (5)
I believe the answer is:
gloss
'lustre' is the definition.
(I've seen this before)
'middle age comes with depletion' is the wordplay.
'middle' indicates the central letters.
'comes with' is a charade indicator (letters next to each other).
'depletion' becomes 'loss'.
The middle of 'age' is 'g'.
'g'+'loss'='GLOSS'
'of' is the link.
(Other definitions for gloss that I've seen before include "High shine" , "Very shiny finish" , "Show" , "marginal note" , "phrase's explanation" .)