Fundamentally, the answer lies in technique (2,5)
I believe the answer is:
at heart
'fundamentally' is the definition.
(I've seen this before)
'the answer lies in technique' is the wordplay.
'answer' becomes 'a' (abbreviation eg in Q and A).
'lies in' indicates putting letters inside.
'technique' becomes 'art' (I've seen this in another clue).
'the'+'a'='thea'
'thea' inserted within 'art' is 'AT HEART'.
(Other definitions for at heart that I've seen before include "In one's innermost feelings, in that era" , "Fundamentally" , "In one's real nature - a threat (anag)" , "In essence" , "Substantially" .)