The most common and most likely answer to this crossword clue is the 5 letter word LUMEN
I've seen this crossword clue in The Daily Telegraph
L
U
M
E
N
Buddy explains!
The straight part
We know that "Light unit measurement’s European norm initially" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case light unit measurement’s european norm initially = lumen.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 LUMEN mean?
LUMEN noun- In the International System of Units, the derived unit of luminous flux; the light that is emitted in a solid angle of one steradian from a source of one candela. Symbol: lm.
- The cavity or channel within a tube or tubular organ.
- The cavity bounded by a plant cell wall.
- The bore of a tube such as a hollow needle or catheter.