Case requires silence about preliminary race (6)
I believe the answer is:
sheath
'case' is the definition.
('sheath' can be a synonym of 'case')
'silence about preliminary race' is the wordplay.
'silence' becomes 'sh' (I've seen this in other clues).
'about' indicates putting letters inside.
'preliminary race' becomes 'heat' (I've seen this before).
'sh' going around 'heat' is 'SHEATH'.
'requires' is the link.
(Other definitions for sheath that I've seen before include "(Blade's) cover" , "Cover for a blade" , "coat" , "Casing for a sword" , "Cover for a knife" .)