2 down: Dominant assembly reportedly leading Lucerne? (5)

I got this out after mistakenly assuming that dominant = alpha because assembly = alp, and for some unbeknownst reason I figured it must be that ha = reportedly leading Lucerne. But in my Googling, my mind expanded upon discovering that lucerne is actually another name for alfalfa, so that dominant assembly = alpha = reportedly leading Lucerne.

24 across: Sparkly Silverchair? (8)

With a cross clue, it was clear that silver = ag and that the remaining six letters would be a kind of chair. What I didn’t know was that a litter is the chair on certain wheelless and human-powered vehicles, and that silver = ag, chair = litter and aglitter = sparkly.

20 down: Lilo’s mate a pain in the long run (6)

I had no idea there was a Disney film called Lilo and Stitch, which is perhaps the reason why I never solved this clue.