The most common and most likely answer to this crossword clue is the 6 letter word ALBEDO
I've seen this crossword clue in Financial Times
A
L
B
E
D
O
More clues leading to the result ALBEDO
Buddy explains!
The straight part
We know that "Measure of reflection" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case measure of reflection = albedo.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 ALBEDO mean?
ALBEDO noun- The fraction of incident light or radiation reflected by a surface or body, commonly expressed as a percentage.
- The whitish inner portion of the rind of citrus fruits that is a source of pectin, commonly referred to as the pith.
- One of the four major stages of the magnum opus, involving purification of the prima materia.