Finally fiddle books? Tax collectors see all! (8)
I believe the answer is:
entirely
'all' is the definition.
(I've seen this before)
'finally fiddle books? tax collectors see' is the wordplay.
'finally' says to take the final letters.
'books?' becomes 'nt' (New Testament).
'tax collectors' becomes 'ir' (abbreviation for Inland Revenue).
'see' becomes 'ely' (see is a place with a cathedral - e.g. Ely).
The final letter of 'fiddle' is 'e'.
'e'+'nt'+'ir'+'ely'='ENTIRELY'
(Other definitions for entirely that I've seen before include "'Fully, wholly (8)'" , "Wholly, totally" , "Wholly and completely" , "Fully or absolutely" , "Without reservation" .)