Basically, two articles on craft (2,5)
I believe the answer is:
at heart
'basically' is the definition.
(both can mean fundamentally or deep down)
'two articles on craft' is the wordplay.
'two articles' can mean two substitutions for 'article'.
'article' becomes 'a' (English indefinite article).
'article' becomes 'the' (the definite article in English).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'craft' becomes 'art' (both can mean cunning or trickery).
'a'+'the'+'art'='AT HEART'
(Other definitions for at heart that I've seen before include "Basically" , "real character" , "A threat, in one's inmost feelings" , "Substantially" , "In one's real nature - a threat (anag)" .)