Spot unwanted garden visitor (4)
I believe the answer is:
mole
This clue is a double definition.
'spot' is the first definition.
(spot or blemish on the skin)
'unwanted garden visitor' is the second definition.
(moles may damage or dig up a garden)
(Other definitions for mole that I've seen before include "A spy within an organisation" , "Tunneller" , "Infiltrator" , "one in the Underworld?" , "Skin blemish and burrowing animal" .)