The most common and most likely answer to this crossword clue is the 6 letter word MUGGLE
I've seen this crossword clue in The Times
M
U
G
G
L
E
Buddy explains!
The straight part
We know that "Potter wasn’t one" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case potter wasn’t one = muggle.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 MUGGLE mean?
MUGGLE noun- (chiefly in the plural) Marijuana.
- A marijuana cigarette; a joint.
- A person who has no magical abilities.
- (by extension) A person who lacks a particular ability or skill; a non-specialist; also, a person who is not a member of a group; an outsider or cowan.
- (geocaching) To deface, destroy, or remove a geocache.
- Often followed by along: to live or work in an unorganized and unplanned way; to muddle along.