Jack Kerouac's novels are very entertaining at first (5)
I believe the answer is:
knave
'jack' is the definition.
('knave' can be a synonym of 'jack')
'kerouac's novels are very entertaining at first' is the wordplay.
'at first' says to take the initial letters.
The initial letters of 'kerouac novels are very entertaining' is 'KNAVE'.
(Other definitions for knave that I've seen before include "cheat of old" , "Yet he may be a diamond" , "Varlet; Jack" , "Jack - scoundrel" , "pack member?" .)