Party-pooper left after presentation of prizes (10)
I believe the answer is:
spoilsport
'party-pooper' is the definition.
(I've seen this before)
'left after presentation of prizes' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'after presentation of' says to put letters next to each other.
'prizes' becomes 'spoils' (spoils can mean prizes or benefits).
'port' after 'spoils' is 'SPOILSPORT'.
(Other definitions for spoilsport that I've seen before include "Wet blanket" , "Party pooper" , "One marring others' pleasure" , "nark" , "Damper" .)