The Republic itself is essentially legitimate (5)
I believe the answer is:
licit
'legitimate' is the definition.
(I know that legitimate can be written as licit)
'the republic itself is essentially' is the wordplay.
'is essentially' says the answer is hidden in the clue (essence can mean the inside of something).
'LICIT' is hidden within 'republic itself'.
(Other definitions for licit that I've seen before include "Legitimate" , "Honest" , "not banned" , "Allowed as by law, not forbidden" , "Sanctioned" .)