The most common and most likely answer to this crossword clue is the 7 letter word PISMIRE
I've seen this crossword clue in Financial Times
P
I
S
M
I
R
E
Buddy explains!
The straight part
We know that "One of 20" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one of 20 = pismire.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 PISMIRE mean?
PISMIRE noun- An ant.