Scour the undergrowth (5)
I believe the answer is:
scrub
This clue is a double definition.
'scour' is the first definition.
(I know that scour can be written as scrub)
'undergrowth' is the second definition.
(scrub can mean undergrowth or vegetation)
'the' is the link.
(Other definitions for scrub that I've seen before include "Clean thoroughly - cancel" , "remove cosmetic" , "Wash with a brush" , "Cancel; clean vigorously" , "something in the garden?" .)