Welcome to the world of Azed, the most accurately clued puzzle regularly published in a British newspaper. However, the accuracy of presentation is not always so high, and in 2,455 10d and 11d are both incorrectly enumerated as (9) when they are in fact (10).
Regarding 10a, you are looking for a five-letter word for merrymaking (more often perhaps seen as a verb) plus a three-letter word for 'boozer' (place not person), the whole lot then being reversed ('knocked over'). The definition is 'Umbellifer'. Hope that helps
I don't know if you already have a copy of the Chambers Dictionary (in paper or electronic form), but it is a huge help when solving barred puzzles such as Azed.