Decaying vegetable matter in Hebridean island (4)
I believe the answer is:
muck
'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" .)