Campaign politically in pub — uproar follows end of oration (9)

'campaign politically' is the definition.
(to barnstorm is to tour an area when campaigning)

'pub uproar follows end of oration' is the wordplay.
'pub' becomes 'bar' (both are drinking establishments).
'uproar' becomes 'storm' (both can mean a commotion).
'follows' means one lot of letters go next to another.
'end of' indicates one should take the final letters.
The final letter of 'oration' is 'n'.
'storm' put after 'n' is 'nstorm'.
'bar'+'nstorm'='BARNSTORM'

'in' is the link.