One stopping riot, perhaps, left with loot (10)
I believe the answer is:
spoilsport
'one stopping riot' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'left with loot' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'with' means one lot of letters go next to another.
'loot' becomes 'spoils' (I've seen this in another clue).
'port' put after 'spoils' is 'SPOILSPORT'.
'perhaps' acts as a link.
Can you help me to learn more?
(Other definitions for spoilsport that I've seen before include "Party pooper" , "One who won't play?" , "Party-pooper" , "Damper" , "One who ruins others' enjoyment" .)