Thick book, perhaps very dry (9)
I believe the answer is:
brainless
'thick' is the definition.
Both the definition and answer are adjectives. Maybe they are linked in a way I don't understand?
'book perhaps very dry' is the wordplay.
I cannot really see how this works, but
'book' could be 'b' (abbreviation for book) and 'b' is found in the answer.
'dry' could be 'rainless' (similar in meaning) and 'rainless' is found 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" .)