Crossword Genius

What they're seeing is even less moisture (5)

Ross

I believe the answer is:

water

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'less moisture' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

'what they're seeing is even' is the wordplay.
'seeing is even' means one should take alternating letters (even-numbered letters are used).
'what'+'theyre'='whattheyre'
The alternate letters of 'whattheyre' are 'WATER'.

Can you help me to learn more?

(Other definitions for water that I've seen before include "Give fluid to (plants)" , "Aqua" , "Inundate" , "Drink - type of 1 across" , "One of the four classical elements" .)

I've seen this clue in the Sydney Morning Herald.
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