Afterthought about angry sea robbers (7)
I believe the answer is:
pirates
'sea robbers' is the definition.
('pirate' can be a synonym of 'sea robber')
'afterthought about angry' is the wordplay.
'afterthought' becomes 'ps' (as in a PS in a letter).
'about' indicates putting letters inside.
'angry' becomes 'irate' (irate can mean angry or annoyed).
'ps' placed around 'irate' is 'PIRATES'.
(Other definitions for pirates that I've seen before include "More than one brigand" , "Chatters, talks foolishly" , "Buccaneers" , "Plagiarists" , "Robbers at sea" .)