Scrub either of two ex-Presidents (4)
I believe the answer is:
bush
'scrub' is the definition.
(I know that scrub can be written as bush)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for bush that I've seen before include "Outback" , "part of shrubbery" , "Shrub; US president" , "Rural African" , "ex-president" .)