Sort of division in political group over idiot mostly (5,4)
I believe the answer is:
party wall
'sort of division' is the definition.
(part of wall in a building)
'political group over idiot mostly' is the wordplay.
'political group' becomes 'party' (a political party).
'over' says to put letters next to each other.
'idiot mostly' becomes 'wall' (I can't justify this - if you can you should give a lot more credence to this answer).
'party'+'wall'='PARTY-WALL'
'in' is the link.
(Other definitions for party wall that I've seen before include "Divider of joined buildings" , "Boundary" , "That shared by two houses" , "may be shared between neighbours" , "Divider common to two rooms or buildings" .)