Mess is where you'll find the brass? (4)
I believe the answer is:
muck
'mess' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for muck that I've seen before include "brass accompaniment?" , "Wet clay, mire" , "Dirt, manure" , "Farmyard manure" , "(Spread) manure" .)