Help get water back to desert (4)
I believe the answer is:
arid
'desert' is the definition.
(thesaurus)
'help get water back' is the wordplay.
'help' becomes 'aid' ('aid' can be a synonym of 'help'**).
'get' indicates putting letters inside.
'back' says to take the final letters.
The final letter of 'water' is 'r'.
'aid' going around 'r' is 'ARID'.
'to' acts as a link.
(Other definitions for arid that I've seen before include "Extremely dry" , "water shortage?" , "Dry raid" , "Sterile" , "Rainless (region)" .)