Piece of designated evidence? (4)
I believe the answer is:
sign
'evidence?' is the definition.
(sign is a kind of evidence)
'piece of designated' is the wordplay.
'piece of' indicates a hidden word.
'SIGN' is hidden in the letters of 'designated'.
(Other definitions for sign that I've seen before include "Public notice; portent" , "Show" , "Put one's name to a document" , "Communicate to the deaf" , "add name" .)