Quiet, angry buccaneer (6)
I believe the answer is:
'buccaneer' is the definition.
(I've seen this before)
'quiet angry' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano').
'angry' becomes 'irate' ('irate' can be similar in meaning to 'angry').
(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" .)