Hello, its a throttle damper or dashpot, designed to slow down the throttle return when de-accelerating to idle, i think to stop wear on the linkage, they're fitted to manual cars, so i assume your car is manual transmission?
Have a read of this and also look at the linkage tour in the tech manual for advice. It would be worth going through the linkage adjustment as well, perhaps the arm to the dampener has come away or was disconnected by a previous owner. There seem to have been a few types, but none of the pictures i've seen look like yours, so it may have been replaced, or perhaps there just isn't a picture of your one in the manual?
Hopefully someone more well informed will offer some advice
https://www.sl113.org/wiki/Fuel/ThrottleI've just done a general search for "throttle damper" or "dashpot" and found lots written about relacements, but still no pictures of one similar to yours