Slow-moving mollusk (5)
I believe the answer is:
snail
'slow-moving mollusk' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for snail that I've seen before include "Proverbially slow-moving mollusc" , "Crawler whose home has a helix pattern" , "Not one of the fleet" , "Sluggish person - kind of mail" , "Slow slithery mollusc" .)