A wet blanket ruins the wine! (10)
I believe the answer is:
spoilsport
'a wet blanket' is the definition.
(I know that wet blanket can be written as spoilsport)
'ruins the wine' is the wordplay.
'ruins' becomes 'spoils' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'the wine' becomes 'port' (port wine is a kind of wine).
'spoils'+'port'='SPOILSPORT'
Can you help me to learn more?
(Other definitions for spoilsport that I've seen before include "Party-pooper" , "nark" , "misery" , "One marring others' pleasure" , "Damper" .)