Something that is not quite a classic, but is still very good and well-liked.
Semiclassic things are usually thought of as being almost as good as classic things, but not quite.
Examples:
The movie is a semiclassic, not quite a classic, but still very good.
The book is a semiclassic, almost a classic, but not quite.
The song is a semiclassic, not as good as a classic, but still very well-liked.
The cake is a semiclassic, not quite a classic dessert, but still very tasty.
The car is a semiclassic, not quite a classic, but still very popular.
The toy is a semiclassic, not quite a classic, but still very fun.