Lustre of grand defeat (5)
I believe the answer is:
gloss
'lustre' is the definition.
(I've seen this before)
'grand defeat' is the wordplay.
'grand' becomes 'g' ('G' can be a synonym of 'grand').
'defeat' becomes 'loss' (I've seen this before**).
'g'+'loss'='GLOSS'
'of' is the link.
(Other definitions for gloss that I've seen before include "Superficial appearance" , "Sheen - paint" , "Shiny type of paint" , "Render plausible" , "Superficial shine" .)