Hi Sam - certainly. Click on the blacked out text for the answers.
12D: Trainee astronaut had no room to spare before partial calcium detox
The definition in the clue is 'Trainee astronaut', which is another way of saying 'Space cadet', but I was a little sneaky in only using half of that phrase as an answer. The wordplay 'had no room to spare before' means lose the first word, 'space', from your answer. The wordplay 'partial calcium detox' tells you to take some of the letters - ca and det - to get the answer. I admit, this clue bent the rules a bit.
16D: Silverback repeatedly goes crazy
The definition is 'crazy'. Silverback misdirects you to think of gorillas. But split the word in two and it's telling you to reverse a word for silver - or in this case its chemical symbol Ag. 'Repeatedly' means put it twice, so you get the answer 'gaga'.
20A: Notice audible shun after x rated red stripper performs first
A tough one! The definition is 'Notice' as a noun, and the answer is 'Attention'. The wordplay is in three parts - one for each syllable - but in reverse order. First 'audible shun' tells you to say shun out loud. This sounds likes sion (as in tension) or tion (as in action). This comes 'after' x - which is the Roman numeral for 'ten'. And finally 'rated red stripper' tells you to strip the letters r e d from rated, leaving you 'at', which performs (goes) first. AT TEN TION!
Oh, okay! these were clever, with both 20A and 12D I got most of the answer but couldn't quite work the whole clue in. 16D I was way off on, but I love using silverback like that!