after seeing his car

[Vik-Nik-Sor]

Hello.

Terrified, Mr Austin did as he was told and then called the police after seeing his car speed off into the distance.

No context, just an example from a dictionary.
"His" here refers not to Mr Austin, but to the person that terrified Mr Austion and then speeded off, am I right?

 

[Rover_KE]

I would take this to mean the bad guy sped off in Mr Austin's car.

What word was being defined for this example?

 

[emsr2d2]

With no further context, I would assume that it was Mr Austin's car. I assume he has been carjacked. He is the only person named in your excerpt so, logically, "his" refers to the one named male.

 

[Vik-Nik-Sor]

What word was being defined for this example?

It's an example of the use of the adverb "off".