Careless, leaving night attire on book (9)
I believe the answer is:
negligent
'careless' is the definition.
(similar in meaning)
'night attire on book' is the wordplay.
'night attire' becomes 'neglige' (I am not sure about this - if you are sure you should believe this answer much more).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'book' becomes 'NT' (New Testament).
'neglige'+'nt'='NEGLIGENT'
'leaving' is the link.
Can you help me to learn more?
(Other definitions for negligent that I've seen before include "sloppy" , "Not minding" , "Of the study of insects" , "Failing to take due care" , "Lax" .)