The most common and most likely answer to this crossword clue is the 5 letter word STILB
S
T
I
L
B
More clues leading to the result STILB
Buddy explains!

The straight part
We know that "Unit" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unit = stilb.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 STILB mean?
STILB noun- A unit of luminance equal to one candela per square centimetre