Robber is quiet and angry (6)
I believe the answer is:
pirate
'robber' is the definition.
(sea-faring robber)
'quiet and angry' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'and' says to put letters next to each other.
'angry' becomes 'irate' (irate can mean angry or annoyed).
'p'+'irate'='PIRATE'
'is' acts as a link.
(Other definitions for pirate that I've seen before include "Hook", for example" , "Run off without permission" , "Sea-robber" , "operating illegally" , "brigand" .)