The most common and most likely answer to this crossword clue is the 6 letter word FILTER
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More clues leading to the result FILTER
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 = filter.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 FILTER mean?
FILTER noun- A device which separates a suspended, dissolved, or particulate matter from a fluid, solution, or other substance; any device that separates one substance from another.
- Electronics or software that separates unwanted signals (for example noise) from wanted signals or that attenuates selected frequencies.
- Any item, mechanism, device or procedure that acts to separate or isolate.
- Self-restraint in speech.
- A non-empty upper set (of a partially ordered set) which is closed under binary infima (a.k.a. meets).
- To sort, sift, or isolate.
- To diffuse; to cause to be less concentrated or focused.
- To pass through a filter or to act as though passing through a filter.
- To move slowly or gradually; to come or go a few at a time.
- To ride a motorcycle between lanes on a road