A plug may be so discovered (9)
I believe the answer is:
unearthed
This clue is a double definition.
'a plug may be so' is the first definition.
(ie not connected with the ground or 'earthed')
'discovered' is the second definition.
(I've seen this before)
(Other definitions for unearthed that I've seen before include "Discovered something hidden by digging" , "Dug up" , "unsafe condition of electrical apparatus" , "Excavated, brought to light" , "found" .)