Wash undergrowth (5)
I believe the answer is:
scrub
I believe this is a double definition.
'wash' is the first definition.
(I know that scrub is a kind of the wash)
'undergrowth' is the second definition.
(scrub can mean undergrowth or vegetation)
(Other definitions for scrub that I've seen before include "Clean thoroughly - cancel" , "Rub hard to clean" , "Clean vigorously - stunted vegetation" , "Scour; cancel" , "Wash with a brush" .)