Formula for making little Sarah's shoe (6)
I believe the answer is:
sandal
'shoe' is the definition.
(I know that sandal is a type of shoe)
I don't understand how the rest of the clue works.
(Other definitions for sandal that I've seen before include "Light open shoe" , "Casual footwear" , "Item of footwear" , "what may be afoot" , "Light strapped shoe" .)