London prison almost clean (5)
I believe the answer is:
scrub
'clean' is the definition.
(scrub can mean to clean)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for scrub that I've seen before include "Cancel (colloq)" , "Drop" , "Take out - clean" , "Clean, low vegetation" , "Rough country area" .)