Essentially new, ground-breaking experience (7)
I believe the answer is:
undergo
'experience' is the definition.
(synonyms)
'essentially new ground-breaking' is the wordplay.
'essentially' indicates the central letters.
'breaking' is an anagram indicator.
The central letter of 'new' is 'e'.
'e'+'ground'='eground'
'eground' with letters rearranged gives 'UNDERGO'.
(Other definitions for undergo that I've seen before include "put up with" , "Endure or be subjected to" , "Withstand" , "Suffer, endure" , "Experience - bear" .)