Book by president is dear (4)
I believe the answer is:
babe
'is dear' is the definition.
(thesaurus)
'book by president' is the wordplay.
'book' becomes 'b'.
'by' says to put letters next to each other.
'president' becomes 'abe' (Abraham (Abe) Lincoln).
'b'+'abe'='BABE'
(Other definitions for babe that I've seen before include "- Ruth, famous US baseball player" , "Stunner" , "baby" , "good-looking young lady" , "Girlfriend" .)