What they're seeing is even less moisture (5)
I believe the answer is:
water
'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" .)