With it this gas could show the extent of your endurance say (5)
I believe the answer is:
ether
'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" .)