The most common and most likely answer to this crossword clue is the 6 letter word WEBERS
W
E
B
E
R
S
Buddy explains!

The straight part
We know that "SI units" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case si units = webers.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 WEBERS mean?
WEBERS noun- In the International System of Units, the derived unit of magnetic flux; the flux linking a circuit of one turn that produces an electromotive force of one volt when reduced uniformly to zero in one second. Symbol: Wb.