Not watered down - honest! (8)

I believe the answer is:

straight

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'honest' is the definition.
(similar in meaning)

I cannot understand how the rest of the clue works.

(Other definitions for straight that I've seen before include "Linear" , "Respectable" , "proper place" , "Honest, not crooked" , "Candid" .)

I've seen this clue in the Evening Standard.

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