Get set (6)
'get set' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for prep that I've seen before include "Homework (abbr.)" , "School work done outside lessons" , "Homework (abbrev.)" , "warm-up race" .)
(Other definitions for harden that I've seen before include "Anneal - solidify" , "Set; become tougher" , "Become less soft" , "Steel oneself" , "Temper, steel say" .)