Pressure on angry buccaneer (6)
I believe the answer is:
pirate
'buccaneer' is the definition.
(historical term for a pirate)
'pressure on angry' is the wordplay.
'pressure' becomes 'p' (scientific abbreviation).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'angry' becomes 'irate' (irate can mean angry or annoyed).
'p'+'irate'='PIRATE'
(Other definitions for pirate that I've seen before include "Corsair" , "Copy published material illegally" , "Sea raider" , "He sailed under the Jolly Roger" , "Captain Hook, say" .)