Corsair quietly getting angry (6)
I believe the answer is:
pirate
'corsair' is the definition.
(I've seen this in another clue)
'quietly getting angry' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'getting' says to put letters next to each other.
'angry' becomes 'irate' (irate can mean angry or annoyed).
'p'+'irate'='PIRATE'
(Other definitions for pirate that I've seen before include "Maritime robber" , "Teach, perhaps" , "Hook for one" , "Freebooter" , "Robber on the high seas" .)