Everyone attending is thinking straight (3,5)

I believe the answer is:

all there

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'everyone attending' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for all there that I've seen before include "Completely sane" , "Everybody attending the event" , "with marbles?" , "rational" , "sensible" .)

I've seen this clue in The Irish Times.

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