Early leaf of nominal importance (5,4)
I believe the answer is:
title page
'nominal importance' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'early leaf' is the wordplay.
'early' becomes 'title' (I can't explain this - if you can you should believe this answer much more).
'leaf' becomes 'page' (page is a kind of leaf).
'title'+'page'='TITLE-PAGE'
'of' is the link.
Can you help me to learn more?
(Other definitions for title page that I've seen before include "One starting book" , "beginning to work" , "Sheet bearing name of book and author" , "appearing at start of play?" , "One of the first leaves" .)