Pens, we're told, ceremonial forms (5)
I believe the answer is:
rites
'ceremonial forms' is the definition.
(I've seen this before)
'pens we're told' is the wordplay.
'pens' becomes 'writes' ('write' can be a synonym of 'pen').
'we're told' shows a homophone (sound like).
'writes' is a homophone of 'RITES'.
(Other definitions for rites that I've seen before include "Social customs" , "Rituals" , "Customary observances, religious say" , "Ceremonial forms" , "Ceremonial observances" .)