Extremist Rees-Mogg elected (7)
I believe the answer is:
jacobin
'extremist' is the definition.
(thesaurus)
'rees-mogg elected' is the wordplay.
'rees-mogg' becomes 'jacob' (politician Jacob Rees-Mogg).
'elected' becomes 'in'.
'jacob'+'in'='JACOBIN'
(Other definitions for jacobin that I've seen before include "Extreme radical" , "pigeon" , "What can make one a wild man" , "French radical" , "French revolutionary" .)