Three articles right, substantially (2,5)
I believe the answer is:
at heart
'substantially' is the definition.
(I've seen this before)
'three articles right' is the wordplay.
'three articles' means three substitutions for 'article'.
'article' becomes 'a' (the indefinite article in English).
'article' becomes 'the' (the definite article in English).
'article' becomes 'a' (the indefinite article in English).
'right' becomes 'rt' (abbreviation as in Rt Hon- the Right Honourable).
'a'+'the'+'a'+'rt'='AT HEART'
(Other definitions for at heart that I've seen before include "real character" , "A threat, in one's inmost feelings" , "In reality" , "In one's real nature - a threat (anag)" , "Really" .)