Cooked slowly (6)
'cooked slowly' is the definition.
('stew' can be a synonym of 'cook slowly')
This is all the clue.
(Other definitions for stewed that I've seen before include "Cooked slowly" , "Was long embarrassed" , "casseroled!" , "felt uneasy" , "Of tea, left brewing too long" .)
(Other definitions for braised that I've seen before include "Fried then slowly stewed" , "Stewed in closed pot" .)