This is Sept 11, 2016.

She was arrested prior to the election????

By whom? Must have been the secret service? but how?

How could she have been arrested while Obama was still president?

Obama as president and chief law enforcement officer could have done whatever he wanted. Executive discretion of law enforcement.
Pardon power.

Some one attempt to explain who Hillary Clinton could have been arrested under Obama's watch before Trump took the reigns of the presidency?

You'd have to say that the military was in control in 2016. I don't see how else she could have been arres