Careless with the dressing gown book (9)
I believe the answer is:
negligent
'careless' is the definition.
(associated in meaning)
'with the dressing gown book' is the wordplay.
'with' says to put letters next to each other.
'the dressing gown' becomes 'neglige' (neglige is a kind of dressing gown).
'book' becomes 'NT' (New Testament).
'neglige'+'nt' is 'NEGLIGENT'.
(Other definitions for negligent that I've seen before include "slack" , "lacking care" , "Careless, remiss" , "sloppy" , "Of the study of insects" .)