Thumb (through) (4)
I believe the answer is:
leaf
'thumb through' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for leaf that I've seen before include "Bit of tree, of book" , "Hinged or removable table flap" , "following 28 [SILVER]" , "Page of a book" , "Piece of foliage (to be turned over anew?)" .)