Administer an oath to
'administer an oath to' is the definition.
(synonyms)
This is all the clue.
(Other definitions for swear in that I've seen before include "Inaugurate by oath" , "Admit to office" , "Administer oath to one taking up office" , "prepare to deliver evidence" , "bring candidate to office" .)
'administer an oath to' is the definition.
(synonyms)
This is all the clue.