One moving slowly with large account book (6)
I believe the answer is:
ledger
'account book' is the definition.
(synonyms)
'one moving slowly with large' is the wordplay.
'one moving slowly' becomes 'edger' (someone edging along slowly).
'with' means one lot of letters go next to another.
'large' becomes 'l'.
'edger' after 'l' is 'LEDGER'.
(Other definitions for ledger that I've seen before include "record-holder" , "Book for keeping financial accounts" , "Old-fashioned heavy accounts book" , "Business record book" , "Old accounts book" .)