Slow-moving mollusc (5)
I believe the answer is:
snail
'slow-moving mollusc' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for snail that I've seen before include "Mollusc with coiled shell" , "Edible terrestrial mollusc" , "Not one of the fleet" , "Soft-bodied garden animal with shell" , "Slow-moving creature" .)