Very grave case of leprosy (9)
I believe the answer is:
seriously
'very' is the definition.
(thesaurus)
'grave case of leprosy' is the wordplay.
'grave' becomes 'serious' (synonyms).
'case of' says to hollow out the word (remove centre letters) (outsides of).
'leprosy' with its centre removed is 'ly'.
'serious'+'ly'='SERIOUSLY'
(Other definitions for seriously that I've seen before include "In a solemn and thoughtful manner" , "Gravely" .)