The most common and most likely answer to this crossword clue is the 2 plus 2 plus 3 letter word DO OR DIE
I've seen this crossword clue in Financial Times
D
O
O
R
D
I
E
More clues leading to the result DO OR DIE
Buddy explains!
The straight part
We know that "Light Brigade’s fatal order, effectively" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case light brigade’s fatal order, effectively = do or die.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 DO OR DIE mean?
DO OR DIE adjective- Requiring a determined or desperate effort to avoid the consequences of failure.