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