Hit a garden pest (4)
I believe the answer is:
slug
I believe this is a double definition.
'hit' is the first definition.
(slugging is a kind of hitting)
'a garden pest' is the second definition.
(I've seen this before)
(Other definitions for slug that I've seen before include "draught" , "Pellet; deal thumping blows" , "Bullet - punch" , "Small creature like snail without shell" , "Terrestrial mollusc" .)