Formality observed when celeb goes to church (6)
I believe the answer is:
starch
'formality' is the definition.
(I've seen this before)
'celeb goes to church' is the wordplay.
'celeb' becomes 'star' (I've seen this before).
'goes to' is a charade indicator (letters next to each other).
'church' becomes 'ch' (common abbreviation).
'star'+'ch'='STARCH'
'observed when' is the link.
(Other definitions for starch that I've seen before include "Setter" , "Stuff that stiffens, say collars" , "Substance found mainly in potatoes and cereals" , "Carbohydrate that occurs in all green plants" , "foodstuff" .)