Thick book, perhaps very dry (9)
I believe the answer is:
brainless
'thick' is the definition.
Both the answer and definition are adjectives. Perhaps there's a link between them I don't understand?
'book perhaps very dry' is the wordplay.
I cannot really understand how this works, but
'book' could be 'b' (abbreviation for book) and 'b' is present in the answer.
'dry' could be 'rainless' (similar in meaning) and 'rainless' is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for brainless that I've seen before include "dim" , "Silly" , "Devoid of intelligence" , "Stupid, foolish" .)