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With it this gas could show the extent of your endurance say (5)

Ross

I believe the answer is:

ether

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'with it this gas could show the extent of your endurance say' is the definition.
'ether' can be an answer for 'gas' (thesaurus). I am unsure of the rest of the definition.

This is all the clue.

Can you help me to learn more?

(Other definitions for ether that I've seen before include "Anaesthetic liquid; clear sky" , "Colourless volatile highly inflammable liquid" , "Colourless liquid used as a solvent" , "Anaesthetic; clear sky (poetic)" , "Liquid used as anaesthetic/ solvent" .)

I've seen this clue in The Sunday Times and The Times.
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