Fop for 12ac put the plural of an elderly relative around a 2 letter abbreviation for a Brussels institution and a letter denoting “one”. Answer is a European nationality (plural).
13d is hard to explain if you don’t know the person, a singer. Festive song should be easy enough for the first five letters, then the last five (loosely) mean getting longer, although I’d usually use the word for making something last longer.
Hope that’s not all too spoilerific