Confident fool initially upsetting socialist (7)
I believe the answer is:
assured
'confident' is the definition.
(similar in meaning)
'fool initially upsetting socialist' is the wordplay.
'fool' becomes 'ass' (both can mean a stupid person).
'initially' suggests taking the first letters.
'socialist' becomes 'red' (red can informally mean a socialist).
The first letter of 'upsetting' is 'u'.
'ass'+'u'+'red'='ASSURED'
(Other definitions for assured that I've seen before include "Secured" , "Confident - certain" , "Informed with certainty and confidence" , "Beyond doubt" , "Sad ruse (anag.)" .)