Wet blanket left on babies (10)
I believe the answer is:
spoilsport
'wet blanket' is the definition.
(synonyms)
'left on babies' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'on' means one lot of letters go next to another.
'babies' becomes 'spoils' ('spoil' can be a synonym of 'baby').
'port' put after 'spoils' is 'SPOILSPORT'.
(Other definitions for spoilsport that I've seen before include "Person marring others' pleasure" , "Killjoy" , "One marring others' pleasure" , "One who ruins others' enjoyment" , "One who reduces others' pleasure" .)