Stupid blockhead; arid is what it is (9)

I believe the answer is:

brainless

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'stupid' is the definition.
(synonyms)

I can't explain the rest of the clue.

(Other definitions for brainless that I've seen before include "Stupid, foolish" , "Devoid of intelligence" , "Silly" , "dim" .)

I've seen this clue in the Evening Standard.

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