Quiet, furious buccaneer (6)
I believe the answer is:
pirate
'buccaneer' is the definition.
(historical term for a pirate)
'quiet furious' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'furious' becomes 'irate' (I've seen this in another clue).
'p'+'irate'='PIRATE'
(Other definitions for pirate that I've seen before include "Criminal on the high seas" , "Plagiarise" , "Silver, perhaps" , "Robber on the high seas" , "Use (another's work) for profit without permission" .)