Dull end to game - but a decisive one! (4)
I believe the answer is:
mate
'a decisive one' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'dull end to game' is the wordplay.
'dull' becomes 'mat' (mat as opposed to glossy).
'end to' suggests the final letters.
The last letter of 'game' is 'e'.
'mat'+'e'='MATE'
'but' is the link.
Can you help me to learn more?
(Other definitions for mate that I've seen before include "Friend, companion" , "tea" , "Fellow worker" , "Pal on team" , "Team (anag.)" .)