A rum and I'd be exceptionally cordial (8)
I believe the answer is:
drambuie
'cordial' is the definition.
(thesaurus)
'a rum and i'd be exceptionally' is the wordplay.
'and' means one lot of letters go next to another.
'exceptionally' indicates anagramming the letters.
'a'+'rum'+'id'+'be'='arumidbe'
'arumidbe' anagrammed gives 'DRAMBUIE'.
(Other definitions for drambuie that I've seen before include "Liqueur based on Scotch whisky" , "short, from Scotland" .)