Essentially, simplicity has to be inferred (8)
I believe the answer is:
implicit
'be inferred' is the definition.
(I've seen this before)
'essentially simplicity has' is the wordplay.
'essentially' says the answer is hidden in the clue (essence can mean the inside of something).
'IMPLICIT' is hidden in the letters of 'simplicity has'.
'to' is the link.
(Other definitions for implicit that I've seen before include "Inherent but not made clear" , "Unquestioning - expressed indirectly" , "Suggested though not directly expressed" , "Absolute and unquestioning - or tacit" , "Absolute and unquestioning, as obedience should be" .)