Applying a filter to labels can be done to mails already in the inbox (filter> containing words> label:label-wanted) but what about incoming emails? That would be really useful, as we can't filter filters, I'm afraid of ending up with a lot of filters doing a lot of specific things.
Filters only ever act in incoming messages. They also seem to be applied in order, so if you've already got a filter applying a label, you may be able to use that label in another label.
Matches: label:label-wanted keyword Do this: ...
You'll get a warning that a filter using "label:" (and some other criteria won't work), but just ignore that.
As long as a previous filter added the label you wanted, and didn't otherwise make the message ineligible, this should work. You'll have to experiment.