Top line on book (4)
I believe the answer is:
brow
'top' is the definition.
(brow is a kind of top)
'line on book' is the wordplay.
'line' becomes 'row' (row is a kind of line**).
'on' is a charade indicator (letters next to each other).
'book' becomes 'b'.
'row' put after 'b' is 'BROW'.
(Other definitions for brow that I've seen before include "Mopped part of the head" , "Top of hill - part of face" , "intellectual level" , "Summit of a hill/pass" , "Forehead or hill summit" .)