Clean undergrowth (5)
I believe the answer is:
scrub
I believe this is a double definition.
'clean' is the first definition.
(scrub can mean to clean)
'undergrowth' is the second definition.
(scrub can mean undergrowth or vegetation)
(Other definitions for scrub that I've seen before include "Clean thoroughly - cancel" , "Wash vigorously" , "poor growth" , "Clean - scrap" , "clean-up" .)