Commercial study has 500 (you say 1000) in a codicil (8)
I believe the answer is:
addendum
'a codicil' is the definition.
(I've seen this in another clue)
'commercial study has 500 you say 1000' is the wordplay.
'commercial' becomes 'ad' (abbreviation for advertisement).
'study has' becomes 'den' (I can't explain this - if you can you should give a lot more credence to this answer).
'500' becomes 'D' (Roman numeral for 500).
'you say' becomes 'u' ('you' as it sounds).
'd000' becomes 'M' (Roman numeral).
'ad'+'den'+'d'+'u'+'m'='ADDENDUM'
'in' is the link.
Can you help me to learn more?
(Other definitions for addendum that I've seen before include "Extra item added at end of book" , "Addition at end of book" , "Extra item added to book or text" , "Piece added on" , "PS" .)