Very big bill occasionally to stuff fabric (8)
I believe the answer is:
terribly
'very' is the definition.
(thesaurus)
'big bill occasionally to stuff fabric' is the wordplay.
'occasionally' means one should take alternating letters.
'to stuff' is an insertion indicator.
'fabric' becomes 'terry' (terry is a kind of fabric**).
'big'+'bill'='bigbill'
The alternating letters of 'bigbill' are 'ibl'.
'ibl' placed into 'terry' is 'TERRIBLY'.
(Other definitions for terribly that I've seen before include "Very badly" , "Awfully, frightfully" , "In an awful manner" , "In a dreadful manner" .)