They are said to be the mirrors of the soul (4)
I believe the answer is:
eyes
'they are said to be the mirrors of the soul' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for eyes that I've seen before include "Organs of vision" , "Narrowly watches" , "Storm centres" , "They have pupils" , "Peepers" .)