Account for one in former scheme (7)
I believe the answer is:
explain
'account for' is the definition.
(thesaurus)
'one in former scheme' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'former' becomes 'ex'.
'scheme' becomes 'plan' (I've seen this before**).
'ex'+'plan'='explan'
'i' going within 'explan' is 'EXPLAIN'.
(Other definitions for explain that I've seen before include "Tell us how" , "Make comprehensible" , "Show the whys and wherefores" , "Defend" , "Resolve" .)