The most common and most likely answer to this crossword clue is the 8 letter word SPARSEST
I've seen this crossword clue in The Guardian
S
P
A
R
S
E
S
T
More clues leading to the result SPARSEST
Buddy explains!

The straight part
We know that "most infrequent" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case most infrequent = sparsest.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 SPARSEST mean?
SPARSEST adjective- Having widely spaced intervals.
- Not dense; meager; scanty
- Having few nonzero elements