Slime spread a long way (5)
I believe the answer is:
miles
'a long way' is the definition.
(I've seen this before)
'slime spread' is the wordplay.
'spread' is an anagram indicator.
'slime' with letters rearranged gives 'MILES'.
(Other definitions for miles that I've seen before include "Distance units" , "'But I have promises to keep, and . . . . . to go before I sleep' (Frost)" , "Imperial units of length" , "a long way" .)