Old company keeping tax returns book (6)
I believe the answer is:
octavo
'book' is the definition.
(I know that octavo is a type of book)
'old company keeping tax returns' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'company' becomes 'co' (abbreviation).
'keeping' indicates putting letters inside (some letters kept inside).
'tax' becomes 'VAT' (Value-Added Tax).
'returns' shows that the letters should be reversed in order.
'o'+'co'='oco'
'oco' going around 'vat' is 'ovatco'.
'ovatco' in reverse letter order is 'OCTAVO'.
(Other definitions for octavo that I've seen before include "Common book size" , "Size of book page" , "An eight-leaved book or page" , "Size of a book, pages are made by folding a sheet of paper three times to form eight leaves (6)" .)