A piece of string, unfortunately (3)
I believe the answer is:
gun
'a piece' is the definition.
(I've seen this before)
'of string unfortunately' is the wordplay.
'of' indicates a hidden word.
'GUN' can be found hidden inside 'string unfortunately'.
(Other definitions for gun that I've seen before include "An offensive weapon" , "What has barrel" , "Arm" , "Hired killer" , "Rev up" .)