The sturgeon's front half may be this colour (4)

I believe the answer is:

rust

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'the sturgeon's front half may be this colour' is the definition.
Both the answer and definition are adjectives. Perhaps you can see a link between them that I don't see?

This is the entire clue.

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(Other definitions for rust that I've seen before include "Reddish-brown coating formed on metal by moisture" , "Reddish oxide coating on iron" , "Oxidisation" , "Fungal disease of plants" , "Reddish-brown coating caused by oxidation" .)

I've seen this clue in the Evening Standard.

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