I'm struggling with this and not enjoying it much.
Is there a spelling mistake in the answer to 13a?
Lilac arrangement extremely pretty and extremely big, not quite having a shapely 5 (11)
It's not giving away too much to say that 5 is BOTTOM - saving grace the "theme" clue was very easy.
I know what sort of word it should be, and there are two alternatives that fit; unfortunately neither has an "i" third last letter (definitely correct).