Formality (6)
I believe the answer is:
starch
'formality' is the definition.
(being excessively formal or stiff)
This is all the clue.
(Other definitions for starch that I've seen before include "Charts (anag) - formality" , "foodstuff" , "Setter" , "Substance found mainly in potatoes and cereals" , "Formal quality" .)