Former college welcomes politician, old American, in grand manner (9)

'in grand manner' is the definition.
(in a grand or majestic manner)

'former college welcomes politician old american' is the wordplay.
'former college' becomes 'poly' (polytechnic).
'welcomes' indicates putting letters inside (in sense of 'invites in').
'politician' becomes 'mp' (Member of Parliament).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'american' becomes 'us' (abbreviation for United States).
'mp'+'o'+'us'='mpous'
'poly' enclosing 'mpous' is 'POMPOUSLY'.