Supply land with water to help growth (8)
I believe the answer is:
irrigate
'supply land with water to help growth' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for irrigate that I've seen before include "Supply water (to crops) in channels" , "Supply land with water to make fertile" , "Provide water to (crops)" , "Bring water to land" , "Provide water artificially" .)