Celebrity on church formality (6)
I believe the answer is:
starch
'formality' is the definition.
(I've seen this before)
'celebrity on church' is the wordplay.
'celebrity' becomes 'star' (I've seen this before).
'on' is a charade indicator (letters next to each other).
'church' becomes 'ch' (common abbreviation).
'star'+'ch'='STARCH'
(Other definitions for starch that I've seen before include "Fabric stiffener" , "Potato, rice constituent; a stiffener" , "It stiffens shirts and collars" , "Carbohydrate found in potatoes, wheat etc." , "Carbohydrate; stiffen" .)