The most common and most likely answer to this crossword clue is the 8 letter word OFTENEST
I've seen this crossword clue in The Guardian
O
F
T
E
N
E
S
T
More clues leading to the result OFTENEST
Buddy explains!
The straight part
We know that "most frequently" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case most frequently = oftenest.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 OFTENEST mean?
OFTENEST adverb- Frequently, many times.