Hose down, say (5)
'hose down say' is the definition.
(hosing down is a kind of irrigating)
This is all the clue.
(Other definitions for irrigate that I've seen before include "Supply land with water to make fertile" , "Bring water to land" , "Supply land with a constant flow of water" , "Supply water (to crops) in channels" , "Provide water artificially" .)
(Other definitions for rinse that I've seen before include "Wash in clean water" , "Wash off surface dirt" , "Swill out" , "Cleanse in water" , "Run water through" .)