Seriously cunning catching mole (foreign nationalist) (8)
I believe the answer is:
solemnly
'seriously' is the definition.
(thesaurus)
'cunning catching mole foreign nationalist' is the wordplay.
'cunning' becomes 'sly' (synonyms).
'catching' indicates putting letters inside.
'foreign' is an anagram indicator (foreign order of the letters).
'nationalist' becomes 'n' (abbreviation in politics).
'mole' is an anagram of 'olem'.
'olem'+'n'='olemn'
'sly' enclosing 'olemn' is 'SOLEMNLY'.
(Other definitions for solemnly that I've seen before include "In a formal and dignified manner" , "Gravely and sedately" , "in formal and dignified way" .)