Extortion, essentially is wrong (4)
I believe the answer is:
tort
'is wrong' is the definition.
(legal term)
'extortion essentially' is the wordplay.
'essentially' indicates the answer is hidden within the clue (essence can mean the inside of something).
'TORT' is hidden within 'extortion'.
(Other definitions for tort that I've seen before include "A legal wrong open to claim for damages" , "Trot after legal offence" , "Legal wrong for which damages may be sought" , "Private or civil wrong - trot after it" , "A private or civil wrong that may attract damages" .)