At heart, fundamentally (2,7)
I believe the answer is:
in essence
This clue is a double definition.
'at heart' is the first definition.
(I've seen this in another clue)
'fundamentally' is the second definition.
(synonyms)
(Other definitions for in essence that I've seen before include "Fundamentally" , "Basically" , "Substantially" , "at bottom" .)