Shelf with foremost of reference books (6)
I believe the answer is:
ledger
'books' is the definition.
(thesaurus)
'shelf with foremost of reference' is the wordplay.
'shelf' becomes 'ledge' ('ledge' can be a synonym of 'shelf').
'with' means one lot of letters go next to another.
'foremost of' says to take the initial letters.
The initial letter of 'reference' is 'r'.
'ledge'+'r'='LEDGER'
(Other definitions for ledger that I've seen before include "record-holder" , "Old accounts book" , "Accounts journal" , "Book-keeper's book" , "In which accounts are entered" .)