Filth that's miles away (5)
I believe the answer is:
slime
'filth' is the definition.
(I've seen this in another clue)
'miles away' is the wordplay.
'away' indicates an anagram (letters move away from their original places).
'miles' with letters rearranged gives 'SLIME'.
'that's' acts as a link.
(Other definitions for slime that I've seen before include "Goo" , "Gooey seeping stuff" , "Gunge" , "could make things sticky" , "Nasty slippery stuff" .)