Garden mollusc (4)
'garden mollusc' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for slug that I've seen before include "Garden mollusc" , "draught" , "Wallop" , "gulp!" , "Terrestrial mollusc" .)
(Other definitions for snail that I've seen before include "10 ac mollusc" , "Slow mover" , "Slow slithery animal" , "Slow-moving creature" , "Not one of the fleet" .)