Sign formal document retaining account (9)
I believe the answer is:
character
'sign' is the definition.
(character is a kind of sign)
'formal document retaining account' is the wordplay.
'formal document' becomes 'charter' (legal document).
'retaining' is an insertion indicator (inserted letters are held or retained).
'account' becomes 'ac' (abbreviation).
'charter' going around 'ac' is 'CHARACTER'.
(Other definitions for character that I've seen before include "Sign" , "Imaginary person in a work of fiction" , "eccentric individual" , "Stamp < letter" , "Amusing individual" .)