The most common and most likely answer to this crossword clue is the 2 plus 1 plus 4 plus 3 letter word BY A LONG WAY
I've seen this crossword clue in The Guardian
B
Y
A
L
O
N
G
W
A
Y
Buddy explains!
The straight part
We know that "to say the least" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to say the least = by a long way.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.