Eye opener in ground breaking experience (7)
I believe the answer is:
undergo
'experience' is the definition.
('undergo' can be a synonym of 'experience')
'eye opener in ground breaking' is the wordplay.
'opener' indicates taking the first letters.
'in' means one lot of letters goes inside another.
'breaking' indicates an anagram.
The first letter of 'eye' is 'e'.
'ground' is an anagram of 'undrgo'.
'e' going inside 'undrgo' is 'UNDERGO'.
(Other definitions for undergo that I've seen before include "Suffer reverse" , "Experience or live through" , "Endure - urged on (anag)" , "Endure for guerdon" , "Suffer, endure" .)