Obscure valley, by the sound of it (4)
I believe the answer is:
veil
'obscure' is the definition.
(I know that obscure can be written as veil)
'valley by the sound of it' is the wordplay.
'valley' becomes 'vale' (synonyms).
'by the sound of it' indicates a 'sounds like' (homophone) clue.
'vale' is a homophone of 'VEIL'.
(Other definitions for veil that I've seen before include "Mantle" , "Face-covering for brides and other women" , "Bridal face covering" , "Conceal, screen" , "Bridal headdress" .)