Decaying vegetable matter in Hebridean island (4)

I believe the answer is:

muck

I'm a little stuck... Click here to teach me more about this clue!

'decaying vegetable matter' is the definition.
'muck' can be an answer for 'matter' (muck is a kind of matter). I'm not certain of the 'decaying vegetable' bit.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for muck that I've seen before include "(Spread) manure" , "Wet clay, mire" , "Dirt (associated with brass)" , "Filth, dirt or slime" , "Dirt, manure" .)

I've seen this clue in The Independent and The Sun.

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