Quiet, angry buccaneer (6)
I believe the answer is:
pirate
'buccaneer' is the definition.
(historical term for a pirate)
'quiet angry' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'angry' becomes 'irate' (irate can mean angry or annoyed).
'p'+'irate'='PIRATE'
(Other definitions for pirate that I've seen before include "Criminal on the high seas" , "Illegal" , "Run off without permission" , "Sea raider" , "Publish without permission" .)