Skill collecting articles deep down (2,5)
I believe the answer is:
at heart
'deep down' is the definition.
(synonyms)
'skill collecting articles' is the wordplay.
'skill' becomes 'art' (both can mean a talent).
'collecting' means one lot of letters goes inside another.
'articles' becomes 'the a' (two grammatical articles).
'art' going around 'thea' is 'AT HEART'.
(Other definitions for at heart that I've seen before include "real character" , "In essence" , "Basically" , "Fundamentally" , "In reality" .)