Simplicity; relief (4)
I believe the answer is:
ease
This clue is a double definition.
'simplicity' is the first definition.
(synonyms)
'relief' is the second definition.
(I know that relief can be written as ease)
(Other definitions for ease that I've seen before include "Become less severe; facilitate" , "At this, one is relaxed" , "soldier may be at it" , "State of relaxation or rest" , "Reduce (pain)" .)