The most common and most likely answer to this crossword clue is the 9 letter word KILOGRAMS
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More clues leading to the result KILOGRAMS
Buddy explains!

The straight part
We know that "mass units" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mass units = kilograms.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 KILOGRAMS mean?
KILOGRAMS noun- In the International System of Units, the base unit of mass; conceived of as the mass of one litre of water, but now defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10-34 when expressed in units of kg⋅m2⋅s−1. Symbol: kg
- The unit of weight such that a one-kilogram mass is also a one-kilogram weight.