Slow slithery mollusc (5)
I believe the answer is:
snail
'slow slithery mollusc' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for snail that I've seen before include "Proverbially slow moving mollusc" , "Mollusc with spiral shell" , "Slow mover" , "Common garden mollusc" , "Slow slithery animal" .)