I think you do have the knack! Lots of good clues in there, very fair, and all the surfaces read nicely. You've got the wrong answer length in the clues for 2D
Plain-clothed, edgy Lorelei seems rather inattentive (9)
Active, wearing crumpled tiara - judicial conclusion "of sound mind" (8)
. The only thing I'm not sure about is the straight definition for 12A
Persist with stretches, until almost completely exhausted (4, 4)
. While it's obvious how it leads to the answer, it feels like the definition is adjective-y, while the answer is a noun phrase. It's a tricky phrase to define. I'm not sure how I would have done it!