Someone legally empowered to witness signatures, certify documents and take depositions (6)
I believe the answer is:
notary
(Other definitions for notary that I've seen before include "Person legally authorised to witness signatures" , "Official working on legal documents" , "Person authorised to witness signatures, etc" , "Solicitor authorised to administer oaths and certify documents" , "Person legally empowered to witness signatures" .)