Unsubstantial passage possibly half a page short (5)

I believe the answer is:

gases

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'unsubstantial passage possibly half a page short' is the definition.
I can't judge whether this defines the answer.

'possibly half a page short' is the wordplay.
'possibly' becomes 'eg' (for example).
'half' suggests halving the letters.
'page short' becomes 'ses' (I can't justify this - if you can you should believe this answer much more).
'eg' with half the letters taken is 'g'.
'g'+'a'+'ses'='GASES'

Can you help me to learn more?

(Other definitions for gases that I've seen before include "Eg, O, H, Ne, CO, etc." , "rabbits" , "Compressible fluids" , "Neon, oxygen, etc" , "Hydrogen and carbon monoxide, say" .)

I've seen this clue in the Evening Standard.

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