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Yes - it is still worrying me. I cant find any representation on the internet of the fifth element that has a horizontal compoment. The figure should consist of vertical and diagonal, but not horizontal lines.

https://www.charliehills.com/media/images/2016/XXXXX-evolution.jpg

is an image showing what you are looking for. Just replace the XXXXX with the correct letters.

https://www.charliehills.com/blog/useless-knowledge-transmission-the-letter-XXXXX/

provides more info I think but my eyelids grew heavy after a couple of sentences

Thanks Jordan - like Smithsax I'd despaired of finding anything corresponding to the very definite graphic produced by the other four thematic items - although I'd surmised it might not be a member of the obvious lists, as the lower vertical component isn't either - only in the looser context of the across message.

But there it is, listed as an evolutionary curiosity.

Not sure whether this makes the final solution devilishly clever, or rather weak.

Being very new to this, can someone please expand on what the “BRB” reference here means? Thank you!

The BRB is simply Chambers dictionary - I htink it's an affectionate term,used often in crossword-splving circles.