The art follows open advocacy in one’s real nature (2,5)
I believe the answer is:
at heart
'in one's real nature' is the definition.
(I've seen this before)
'the art follows open advocacy' is the wordplay.
'follows' says to put letters next to each other.
'open' indicates taking the first letters (take the opening letter).
The first letter of 'advocacy' is 'a'.
'the'+'art'='theart'
'theart' after 'a' is 'AT HEART'.
(Other definitions for at heart that I've seen before include "Fundamentally" , "A threat, in one's inmost feelings" , "real character" , "In one's innermost feelings, in that era" , "Essentially" .)