Killjoy goes off dessert wine (10)
I believe the answer is:
spoilsport
'killjoy' is the definition.
(I know that killjoy can be written as spoilsport)
'off dessert wine' is the wordplay.
'off' becomes 'spoils' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'dessert wine' becomes 'port' (I've seen this in another clue).
'spoils'+'port'='SPOILSPORT'
'goes' acts as a link.
Can you help me to learn more?
(Other definitions for spoilsport that I've seen before include "Wet blanket" , "Person marring others' pleasure" , "misery" , "One who won't play?" , "One marring others' pleasure" .)