The most common and most likely answer to this crossword clue is the 7 letter word CLYSTER
I've seen this crossword clue in The Guardian
C
L
Y
S
T
E
R
Buddy explains!
The straight part
We know that "It will purge 1 down" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it will purge 1 down = clyster.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 CLYSTER mean?
CLYSTER noun- A medicine applied via the rectum; an enema or suppository.