Spot unwanted visitor in the garden (4)
I believe the answer is:
mole
I believe this clue is a double definition.
'spot' is the first definition.
(spot or blemish on the skin)
'unwanted visitor in the garden' is the second definition.
(moles may damage or dig up a garden)
(Other definitions for mole that I've seen before include ""Gentleman in black velvet''" , "Furry burrowing animal" , "sauce" , "Jetty; animal" , "spy a > breakwater" .)