Slow-moving slugs (6)
I believe the answer is:
snails
'slow-moving slugs' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for snails that I've seen before include "Garden creatures" , "Soft-bodied animals with spiral shells, common in garden" , "Edible gastropods" , "'To the French, they're escargots (6)'" , "Slow-moving garden molluscs" .)