In reality, deep down (2,5)
I believe the answer is:
at heart
I believe this clue is a double definition.
'in reality' is the first definition.
(I've seen this before)
'deep down' is the second definition.
(synonyms)
(Other definitions for at heart that I've seen before include "In one's innermost feelings, in that era" , "Basically" , "In essence" , "Really" , "Essentially" .)