Account for one taken in by former scheme (7)
I believe the answer is:
explain
'account for' is the definition.
(I've seen this before)
'one taken in by former scheme' is the wordplay.
'one' becomes 'i' (Roman numeral).
'taken in by' is an insertion indicator.
'former' becomes 'ex' (prefix meaning former or onetime).
'scheme' becomes 'plan' (synonyms).
'ex'+'plan'='explan'
'i' going into 'explan' is 'EXPLAIN'.
(Other definitions for explain that I've seen before include "Interpret" , "Show the whys and wherefores" , "Make intelligible and account for" , "Describe" , "Justify" .)