Put on a finish to a literary supplement (7)
I believe the answer is:
addenda
'literary supplement' is the definition.
'addenda' can be an answer for 'supplement' (I've seen this before). I'm unsure of the 'literary' bit.
'put on a finish to a' is the wordplay.
'put on' becomes 'add' (as in adding something on).
'a finish' becomes 'end' (come to an end).
'add'+'end'+'a'='ADDENDA'
Can you help me to learn more?
(Other definitions for addenda that I've seen before include "Additions to documents" , "Postscripts, supplements" , "Textual supplements" , "Items added to a text" , "Things added to the end of books" .)