Nick quietly angry (6)
I believe the answer is:
pirate
'nick' is the definition.
(both can mean to steal)
'quietly angry' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'angry' becomes 'irate' (irate can mean angry or annoyed).
'p'+'irate'='PIRATE'
(Other definitions for pirate that I've seen before include "Bandit of the ocean, buccaneer" , "Robber on the high seas" , "Copyright infringer" , "Criminal on the high seas" , "Maritime robber" .)