I've finished, even thought some of the clues are frustrating and I'm not sure they all work. However, it is a clever double take on the phrase in the middle.
17a - thoughts = mentally (which I don't think is correct). Circling means split the word and change the order to get people who sell things on tick.
8a is not in Chambers, but it is if you put AN in front of the answer.
And, marca, the thematic down answers are not all in one half.