Formality of celebrity and Companion of Honour (6)
I believe the answer is:
starch
'formality' is the definition.
(being excessively formal or stiff)
'celebrity and companion of honour' is the wordplay.
'celebrity' becomes 'star' (I've seen this before).
'and' says to put letters next to each other.
'companion of honour' becomes 'CH' (abbreviation).
'star'+'ch'='STARCH'
'of' acts as a link.
(Other definitions for starch that I've seen before include "Formal quality" , "Stuff that stiffens, say collars" , "Stiffness of manner" , "Fabric stiffener" , "Carbohydrate found in potatoes, wheat etc." .)