They're said to be the mirrors of the soul (4)
I believe the answer is:
eyes
'they're said to be the mirrors of the soul' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for eyes that I've seen before include "Visual organs" , "Narrowly watches" , "They're said to be the mirrors of the soul" , "Storm centres" , "Views" .)