Rest in peace as expired (4)
I believe the answer is:
ease
'rest' is the definition.
(I know that rest can be written as ease)
'in peace as expired' is the wordplay.
'in' indicates a hidden word.
'EASE' can be found hidden inside 'peace as expired'.
(Other definitions for ease that I've seen before include "Freedom from worry" , "Reduce (pain)" , "Calm" , "Leisure, comfort" , "Affluence" .)