Quietly furious one's stealing from work (6)
I believe the answer is:
pirate
'one's stealing from work' is the definition.
I can't tell whether this definition defines the answer.
'quietly furious' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'furious' becomes 'irate' (I've seen this before).
'p'+'irate'='PIRATE'
Can you help me to learn more?
(Other definitions for pirate that I've seen before include "Maritime robber" , "Hook for one" , "Criminal on the high seas" , "Silver, perhaps" , "One who infringes another's copyright" .)