Ghastly game's ending in mire (5)
I believe the answer is:
grime
'mire' is the definition.
(thesaurus)
'ghastly game's ending' is the wordplay.
'ghastly' becomes 'grim' (I've seen this before).
'ending' suggests the final letters.
The last letter of 'game' is 'e'.
'grim'+'e'='GRIME'
'in' acts as a link.
(Other definitions for grime that I've seen before include "Engrained dirt" , "Muck" , "Dirt, grease" , "Soot" , "Ingrained dirt" .)