Fitted up celebrated Republican on board (6)
I believe the answer is:
framed
'fitted up' is the definition.
The answer and definition can be both to do with fundamental existence as well as being past participle verbs.
Perhaps there's an association between them I don't understand?
'celebrated republican on board' is the wordplay.
'celebrated' becomes 'famed' (synonyms).
'republican' becomes 'r' (abbreviation).
'on board' means one lot of letters goes inside another.
'famed' placed around 'r' is 'FRAMED'.
Can you help me to learn more?
(Other definitions for framed that I've seen before include "Enclosed" , "Incriminated by false evidence" , "In slang, made a victim of false evidence" , "Caught in web of false evidence" , "(Of an picture) having a border right round" .)