Good man raised idea, half foolish (6)
I believe the answer is:
stupid
'foolish' is the definition.
(synonyms)
'good man raised idea half' is the wordplay.
'good man' becomes 'st' (abbreviation for saint).
'raised' becomes 'up' (similar in meaning).
'half' indicates taking half.
'idea' halved is 'id'.
'st'+'up'+'id'='STUPID'
(Other definitions for stupid that I've seen before include "Unintelligent, really thick" , "potty" , "Ass" , "Without much intelligence" , "Dim" .)