Stiffener celebrity required before church (6)
I believe the answer is:
starch
'stiffener' is the definition.
(starch is used to make fabric stiff)
'celebrity required before church' is the wordplay.
'celebrity' becomes 'star' (I've seen this in another clue).
'required before' says to put letters next to each other.
'church' becomes 'ch' (common abbreviation).
'star'+'ch'='STARCH'
(Other definitions for starch that I've seen before include "Carbohydrate; stiffen" , "Fabric stiffener" , "Stiffness; carbohydrate" , "rigid" , "Stuff that stiffens, say collars" .)