What exactly, is this Pasuk coming to teach us?
Rashi: If you fear Hashem, serve Hashem and cling to Him, then and only then you may swear by His Name.
Why does the Torah add "u'Vo Sidbak" here, but not above (6:13)?
Refer to 6:13:151:1-5.
What is clingng to Hashem?
Refer to 11:22:4:1-7.