The game's nearly up, I see, but it's wonderful! (5)
I believe the answer is:
magic
'it's wonderful' is the definition.
(I've seen this before)
'the game's nearly up i see' is the wordplay.
'nearly' means to remove the last letter.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'see' becomes 'C' (the word for the letter, according to Chambers).
'game' with its last letter taken away is 'gam'.
'gam' back-to-front is 'mag'.
'mag'+'i'+'c'='MAGIC'
'but' is the link.
(Other definitions for magic that I've seen before include "'Charms, spells, illusions (5)'" , "Supernatural power" , "Unexpectedly produce" , "Marvellous" , "Great" .)