Pest is lying in shade (8)
I believe the answer is:
Here is my best explanation:
'pest' is the definition.
'is lying in shade' is the wordplay.
'lying in' means one lot of letters goes inside another.
'shade' becomes 'nuance' ('nuance' can be a synonym of 'shade').
'is' going inside 'nuance' is 'NUISANCE'.
(Other definitions for nuisance that I've seen before include "Annoying thing/person", "See 16", "A source of irritation", "One causing annoyance", "Irritation or pest", "Pain in the neck, pest", "Annoying or troublesome person or thing", "Pest, source of annoyance", "Troublesome and annoying person" and "Pest or irritation".)