Despair of game, being pinned by Grand Master (5)
I believe the answer is:
gloom
'despair' is the definition.
(thesaurus)
'game being pinned by grand master' is the wordplay.
'game' becomes 'loo' (historic card game found in Jane Austen novels).
'being pinned by' means one lot of letters goes inside another.
'grand' becomes 'g' (G is short for grand as in a thousand).
'master' becomes 'm'.
'g'+'m'='gm'
'loo' going within 'gm' is 'GLOOM'.
'of' acts as a link.
(Other definitions for gloom that I've seen before include "Dimness" , "Melancholy state" , "Cloudiness" , "Dark depression?" , "Depression and melancholy" .)