Various miles of thin mud (5)
I believe the answer is:
slime
'thin mud' is the definition.
(I've seen this in another clue)
'various miles' is the wordplay.
'various' indicates anagramming the letters.
'miles' is an anagram of 'SLIME'.
'of' is the link.
(Other definitions for slime that I've seen before include "Moist slippery stuff" , "could make things sticky" , "Nasty slippery stuff" , "Sludge, gloop" , "Gooey seeping stuff" .)