Stylish, maybe, but look out for a bad case of rot! (6)

I believe the answer is:

sporty

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'stylish' is the definition.
(I've seen this in another clue)

I cannot understand how the remainder of the clue works.

(Other definitions for sporty that I've seen before include "Given to physical game-playing" , "Energetic" , "preferring to play" , "Flashy" , "This 13 [SPICE]" .)

I've seen this clue in the Evening Standard.

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