In plain terminology, lay to rest (5)
I believe the answer is:
inter
'lay to rest' is the definition.
('inter' can be a synonym of 'lay to rest')
'in plain terminology' is the wordplay.
'in' says the answer is hidden in the clue.
'INTER' can be found hidden inside 'plain terminology'.
(Other definitions for inter that I've seen before include "Place in ground" , "Put underground" , "Put into the ground" , "an Italian soccer team?" , "Football Club seen in Milan" .)