Bother about church's formality (6)
I believe the answer is:
starch
'formality' is the definition.
(being excessively formal or stiff)
'bother about church's' is the wordplay.
'bother' becomes 'rats' (exclamation of frustration).
'about' says the letters should be written backwards.
'church' becomes 'ch' (common abbreviation).
'rats' in reverse letter order is 'star'.
'star'+'ch'='STARCH'
(Other definitions for starch that I've seen before include "Stiffness; carbohydrate" , "Formality - makes things stiff" , "Carbohydrate that occurs in all green plants" , "rigid" , "Substance found mainly in potatoes and cereals" .)