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