Most shady garment the writer set out (9)

I believe the answer is:

shiftiest

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'most shady' is the definition.
Both the definition and answer are superlative adjectives. Perhaps there's an association between them I don't understand?

'garment the writer set out' is the wordplay.
'garment' becomes 'shift' (shift is a kind of garment).
'the writer' becomes 'I' (the person who is writing these clues).
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'set' anagrammed gives 'est'.
'shift'+'i'+'est'='SHIFTIEST'

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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