Formality of a celebrity going to church (6)
I believe the answer is:
starch
'formality' is the definition.
(being excessively formal or stiff)
'celebrity going to church' is the wordplay.
'celebrity' becomes 'star' (a star is a celebrity or famous person).
'going to' says to put letters next to each other.
'church' becomes 'ch' (common abbreviation).
'star'+'ch'='STARCH'
'of a' acts as a link.
(Other definitions for starch that I've seen before include "Stiffening agent" , "rigid" , "Stiffener in the laundry" , "It stiffens shirts and collars" , "Formality - makes things stiff" .)