The most common and most likely answer to this crossword clue is the 5 letter word LEACH
I've seen this crossword clue in Independent
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Buddy explains!
The straight part
We know that "Drain" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case drain = leach.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does LEACH mean?
LEACH noun- A quantity of wood ashes, through which water passes, and thus imbibes the alkali.
- A tub or vat for leaching ashes, bark, etc.
- A jelly-like sweetmeat popular in the fifteenth century.
- An aquatic blood-sucking annelid of class Hirudinea, especially Hirudo medicinalis.
- A person who derives profit from others in a parasitic fashion.
- A glass tube designed for drawing blood from damaged tissue by means of a vacuum.
- A physician.
- (Heathenry) A healer.
- The vertical edge of a square sail.
- The aft edge of a triangular sail.
- To purge a soluble matter out of something by the action of a percolating fluid.
- To part with soluble constituents by percolation.