Unite political party, terribly isolated (11)
I believe the answer is:
consolidate
'unite' is the definition.
(consolidating is a kind of uniting)
'political party terribly isolated' is the wordplay.
'political party' becomes 'con'.
'terribly' indicates an anagram.
'isolated' anagrammed gives 'solidate'.
'con'+'solidate'='CONSOLIDATE'
(Other definitions for consolidate that I've seen before include "Cement" , "Make stronger; combine into one" , "Make firm" , "Unite" , "Unify" .)