I'm nested messily in river mud (8)
I believe the answer is:
sediment
'river mud' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'i'm nested messily in' is the wordplay.
'messily' is an anagram indicator.
'in' indicates putting letters inside.
'nested' anagrammed gives 'sedent'.
'im' going inside 'sedent' is 'SEDIMENT'.
Can you help me to learn more?
(Other definitions for sediment that I've seen before include "Residue" , "Deposit at the bottom of a liquid" , "Grounds - debris" , "Matter settling (on seabed)" , "Lees" .)