Garden mollusk (5)
I believe the answer is:
snail
'garden mollusk' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for snail that I've seen before include "Slow-moving garden pest" , "Gastropod mollusc" , "Slow-moving 12" , "dawdler" , "10 ac mollusc" .)