Substitute for Calvados, reduced cider?
One tablespoon of apple brandy in a recipe of that volume is an accent; it may be nice to have, but it is not going to dramatically transform the outcome.

The obvious choice is to simply omit it.

Otherwise, some reasonable substitutions include:


Vodka.  While it doesn't bring a specific flavor, it will provide alcohol to help dissolve those flavor components that otherwise would not be as apparent, perhaps intensifying the flavor of the cake.
Bourbon.  Compliments the vanilla, and provides a counterpoint to the apples.
Brandy, either grape or another fruit like pear.  Another, similar flavor accent.


There is little point in reducing soft cider because it will not contain alcohol, and the quantity is not enough to matter, unless (and then only maybe) you make a syrup--in that case, however, you would be more substantially transforming the recipe.

Hard cider has some alcohol, so might work (after all, Calvados or apple brandy is distilled hard cider), but you don't want to reduce it, which would drive a significant percentage of the alcohol and volatile flavor compounds.   Freeze-concentrating it (as applejack was once made) would work but is probably excessive in terms of effort and reward.