Firm passes on many received manuscripts (7)
I believe the answer is:
codices
'manuscripts' is the definition.
(codex is a kind of manuscript)
'firm passes on many received' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'passes on' becomes 'dies' (passes away).
'many' becomes 'c' (C is the Roman numeral for 100).
'received' is an insertion indicator.
'dies' enclosing 'c' is 'dices'.
'co'+'dices'='CODICES'
(Other definitions for codices that I've seen before include "ancient documents" , "Old manuscript books" , "Ancient manuscript books" , "Ancient manuscripts" , "Volumes" .)