Wet blanket left with proceeds of robbery, say (10)
I believe the answer is:
spoilsport
'wet blanket' is the definition.
(synonyms)
'left with proceeds of robbery say' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'with' is a charade indicator (letters next to each other).
'proceeds of robbery say' becomes 'spoils' (I can't explain this - if you can you should give a lot more credence to this answer).
'port' put after 'spoils' is 'SPOILSPORT'.
Can you help me to learn more?
(Other definitions for spoilsport that I've seen before include "One marring others' pleasure" , "misery" , "Person marring others' pleasure" , "nark" , "One who won't play?" .)