Pest is lying in shade (8)
I believe the answer is:
nuisance
'pest' is the definition.
(both can mean an annoying person)
'is lying in shade' is the wordplay.
'lying in' means one lot of letters goes inside another.
'shade' becomes 'nuance' (synonyms).
'is' inserted into 'nuance' is 'NUISANCE'.
(Other definitions for nuisance that I've seen before include "Something which annoys" , "Bother" , "Bothersome annoying person" , "Source of annoyance or aggravation" , "Annoying or troublesome person or thing" .)