Justify bringing one into old scheme (7)
I believe the answer is:
explain
'justify' is the definition.
(explaining is a kind of justifying)
'one into old scheme' is the wordplay.
'one' becomes 'i' (Roman numeral).
'into' is an insertion indicator.
'old' becomes 'ex' (prefix meaning former or onetime).
'scheme' becomes 'plan' (synonyms).
'ex'+'plan'='explan'
'i' going within 'explan' is 'EXPLAIN'.
'bringing' acts as a link.
(Other definitions for explain that I've seen before include "Shed light" , "offer clarification" , "Interpret" , "Elucidate, make clear" , "Resolve" .)