Margin of book, its left edge on top (6)
I believe the answer is:
border
'top' is the definition.
(thesaurus)
'margin of book' is the wordplay.
'margin of' becomes 'b' (I can't justify this - if you can you should believe this answer much more).
'book' becomes 'order' (order book is a kind of book).
'b'+'order'='BORDER'
'its left edge on' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for border that I've seen before include "Rim" , "Frontier or edge as of garden" , "Edge or boundary" , "Line between countries" , "Type of sheepdog" .)