Dye originally worn as decoration, hiding nothing (4)
I believe the answer is:
woad
'dye' is the definition.
(I've seen this before)
'originally worn as decoration hiding nothing' is the wordplay.
'originally' indicates taking the first letters.
'hiding' indicates putting letters inside.
'nothing' becomes 'o' (looks like zero - 0).
The initial letters of 'worn as decoration' is 'wad'.
'wad' going around 'o' is 'WOAD'.
(Other definitions for woad that I've seen before include "Old British blue dye" , "Blue dyestuff" , "Ancient blue dye" , "Dye used by ancient Britons" , "Plant providing blue dye" .)