The most common and most likely answer to this crossword clue is the 7 letter word CANDELA
I've seen this crossword clue in The Times
C
A
N
D
E
L
A
More clues leading to the result CANDELA
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The straight part
We know that "Measure of brightness" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case measure of brightness = candela.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 CANDELA mean?
CANDELA noun- In the International System of Units, the base unit of luminous intensity; the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. Symbol: cd