“Hates Crooks,” a hard cover (6)
I believe the answer is:
sheath
'cover' is the definition.
(I know that sheath is a type of cover)
'hates crooks a hard' is the wordplay.
'crooks' indicates an anagram (I've seen 'crook' mean this (a crooked or broken form of the letters)).
'a hard' becomes 'h' (abbreviation used in pencil classifications).
'hates' with letters rearranged gives 'sheat'.
'sheat'+'h'='SHEATH'
(Other definitions for sheath that I've seen before include "Casing for a sword" , "Protective case for a knife or sword" , "Condom" , "(Blade's) cover" , "Sword covering" .)