Only partially clean a dirty bottom (5)
I believe the answer is:
nadir
'bottom' is the definition.
(I've seen this before)
'only partially clean a dirty' is the wordplay.
'only partially' says the answer is hidden in the clue.
'NADIR' is hidden in the letters of 'clean a dirty'.
(Other definitions for nadir that I've seen before include "All-time low" , "Given lowest place" , "least successful point" , "Low-water mark" , "The lowest point of anything" .)