Woman ruined hat cover (6)
I believe the answer is:
sheath
'cover' is the definition.
(I know that sheath is a type of cover)
'woman ruined hat' is the wordplay.
'woman' becomes 'she' ().
'ruined' indicates an anagram.
'hat' anagrammed gives 'ath'.
'she'+'ath'='SHEATH'
(Other definitions for sheath that I've seen before include "Protective covering for a knife" , "cocktail dress" , "Covering for blade" , "Casing for a sword" , "(Blade's) cover" .)