Plunder harbour causing misery (10)
I believe the answer is:
spoilsport
'misery' is the definition.
(I've seen this before)
'plunder harbour' is the wordplay.
'plunder' becomes 'spoils' (I've seen this before).
'harbour' becomes 'port' (I've seen this before).
'spoils'+'port'='SPOILSPORT'
'causing' is the link.
(Other definitions for spoilsport that I've seen before include "misery" , "Wet blanket" , "Party-pooper" , "One who reduces others' pleasure" , "One who ruins others' enjoyment" .)