Just clever, having no need of book (5)
I believe the answer is:
right
'just' is the definition.
(similar in meaning)
'clever having no need of book' is the wordplay.
'clever' becomes 'bright' (I've seen this before).
'having no need of' indicates named letters should be taken away.
'book' becomes 'b'.
'bright' with 'b' taken out is 'RIGHT'.
(Other definitions for right that I've seen before include "Just - correct - title" , "Starboard side" , "one side" , "straight" , "Correct; political wing" .)