How it works...

We declare a UPROPERTY to store our spawned Actor instance, and a custom function so that we can call Destroy() on a timer:

UPROPERTY() 
AMyFirstActor* SpawnedActor; 
UFUNCTION() 
void DestroyActorFunction(); 

In BeginPlay, we assign the spawned Actor to our new UPROPERTY:

SpawnedActor = GetWorld()->SpawnActor<AMyFirstActor> 
 (AMyFirstActor::StaticClass(), SpawnLocation);

We then declare a TimerHandle object and pass it to GetWorldTimerManager::SetTimer. SetTimer calls DestroyActorFunction on the object that was pointed to by this pointer after 10 seconds. SetTimer returns an object – a handle – to allow us to cancel the timer if necessary. The SetTimer function takes the TimerHandle object in as a reference parameter, and so we declare it in advance so that we can pass it into the function properly, even if we aren't going to be using it again:

FTimerHandle Timer; 
GetWorldTimerManager().SetTimer(Timer, this, 
 &AUE4CookbookGameMode::DestroyActorFunction, 10);

DestroyActorFunction checks whether we have a valid reference to a spawned Actor:

void AUE4CookbookGameMode::DestroyActorFunction() 
{ 
  if (SpawnedActor != nullptr) 
  {
     // Then we know that SpawnedActor is valid
  }
} 

If we do, it calls Destroy on the instance so it will be destroyed and, eventually, garbage-collected:

SpawnedActor->Destroy();