What does the words 'clocked'and 'flat' in this sentence means ?

"The cheetahs are known for their speed. The record for the fastest run is held by Sarah who lives at the Cincinnati zoo and Botanical Garden. She clocked her chasing a stuffed dog across a field in 6.16 seconds flat"

What does the words 'clocked' and 'flat' in this sentence means ?

Thanks in advance.