Plunderer’s damage poet briefly repaired to begin with (8)
I believe the answer is:
marauder
'plunderer's' is the definition.
(thesaurus)
'damage poet briefly repaired to begin with' is the wordplay.
'damage' becomes 'mar' (to mar is to spoil or damage).
'poet briefly' becomes 'aude' (I can't explain this - if you can you should give a lot more credence to this answer).
'to begin with' suggests taking the first letters.
The initial letter of 'repaired' is 'r'.
'mar'+'aude'+'r'='MARAUDER'
(Other definitions for marauder that I've seen before include "One going about stealing or attacking" , "person in search of prey and plunder" , "One wandering in search of prey or loot" , "Raider" , "Brigand" .)