Drains baths from the bottom (4)
I believe the answer is:
saps
'drains' is the definition.
(sapping is a kind of draining)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for saps that I've seen before include "Weakens, undermines" , "trenches" , "Pass around foolish people" , "Enervates in spas" , "mugs" .)