Helium in water changing climate (7)
I believe the answer is:
weather
'climate' is the definition.
(weather is a kind of climate)
'helium in water changing' is the wordplay.
'helium' becomes 'he' (chemical symbol).
'in' indicates putting letters inside.
'changing' is an anagram indicator.
'water' is an anagram of 'weatr'.
'he' put inside 'weatr' is 'WEATHER'.
(Other definitions for weather that I've seen before include "Survive (ordeal); be worn away naturally" , "Climate" , "Withstand" , "Come safely through; erode" , "Endure" .)