Formal document containing a Catholic's letter (9)
I believe the answer is:
character
'letter' is the definition.
(written character)
'formal document containing a catholic's' is the wordplay.
'formal document' becomes 'charter' (legal document).
'containing' is an insertion indicator.
'catholic' becomes 'C' (abbreviation as in RC for Roman Catholic).
'a'+'c'='ac'
'charter' enclosing 'ac' is 'CHARACTER'.
(Other definitions for character that I've seen before include "eccentric individual" , "Amusing individual" , "constitution" , "Letter; personality" , "Type - letter" .)