The most common and most likely answer to this crossword clue is the 5 letter word SETON
S
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Buddy explains!
The straight part
We know that "drainage flow" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case drainage flow = seton.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 SETON mean?
SETON noun- A few silk threads or horsehairs, or a strip of linen etc., introduced beneath the skin by a knife or needle, so as to induce suppuration; also, the issue so formed.