Reactionary person in two parties (4)
I believe the answer is:
dodo
'reactionary' is the definition.
(thesaurus)
'two parties' is the wordplay.
'two parties' means two replacements for 'party'.
'party' becomes 'do' (do can informally mean a party).
'party' becomes 'do' (do can informally mean a party).
'do'+'do'='DODO'
'person in' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for dodo that I've seen before include "Two rooks - one was unable to fly" , "Idiot" , "one finished?" , "Ultra-conservative" , "Old bird" .)