Thick mud (4)
I believe the answer is:
mire
'thick mud' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mire that I've seen before include "Deep soft mud" , "Bog down in swamp" , "Emir found in mud" , "Swampy ground" , "(Stretch of) clinging mud" .)