Slimy mud (4)
I believe the answer is:
ooze
'slimy mud' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ooze that I've seen before include "Flow out slowly or thickly" , "... sweat" , "River-bed mud; leak slowly" , "Seep, exude" , "Percolate" .)