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I've done 4723 and 4724 in the last few days, catching up!
Should have saved one as it's numerical this week!
In 4723 I'm all done except still can't get 5a, if anyone can remember to give me a steer.
I even know how it has to change in final step, but can't get it. I have theme etc.
5a - Series about a Greek character returning. (6) C????A
Earlier posts didn't help sorry! I think two clashes?

It’s a one letter abbreviation for about, followed by “a Greek letter” reversed. The letter begins with a vowel. Answer is a Latin word for chain

Thank you quisling, especially for mentioning the vowel.
Pretty tough, solution and wordplay - with two clashes!