Nature of account in document (9)
I believe the answer is:
character
'nature' is the definition.
(I've seen this before)
'account in document' is the wordplay.
'account' becomes 'ac'.
'in' means one lot of letters goes inside another.
'document' becomes 'charter' (charter is a kind of document**).
'ac' put within 'charter' is 'CHARACTER'.
'of' acts as a link.
(Other definitions for character that I've seen before include "constitution" , "Person in a play; letter" , "Personal moral qualities" , "eccentric individual" , "crank" .)