Simplicity of sense is a hit (8)
I believe the answer is:
easiness
'simplicity' is the definition.
(synonyms)
'sense is a hit' is the wordplay.
'hit' indicates an anagram.
'sense'+'is'+'a'='senseisa'
'senseisa' with letters rearranged gives 'EASINESS'.
'of' acts as a link.
(Other definitions for easiness that I've seen before include "Nonchalance" , "Facile quality" , "Lack of difficulty" .)