Content in schedule affected - that's one for the books (4)
I believe the answer is:
Here is my best explanation:
'one for the books' is the definition.
I can't tell whether this defines the answer.
'content in schedule affected' is the subsidiary indication.
'content' indicates the answer is hidden within the clue (I've seen 'content of' mean this).
'leaf' can be found hidden inside 'in schedule affected'.
'that's' is the link.
(Other definitions for leaf that I've seen before include "Foil", "Frond", "Hinged or removable table flap", "See 1", "Page; piece of tree", "Growth on plant", "bit of greenery", "Foliage; page", "Tree part - page" and "Turn over a new one to reform".)