Lean brainbox who collaborated with Watson heard from Spooner, with mystery (7)

I believe the answer is:

cryptic

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'lean brainbox who collaborated with watson heard from spooner with mystery' is the definition.
Both the definition and answer are adjectives. Maybe they are linked in a way I don't understand?

This is the entire clue.

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(Other definitions for cryptic that I've seen before include "Sort of crossword" , "this style?" , "Mysteriously obscure" , "Type of crossword clue (but not this one!)" , "(Of crossword) withindirect clues" .)

I've seen this clue in The Independent.

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