Well acquainted with the metric system (6)
I believe the answer is:
versed
'well' is the definition.
Both the definition and answer are adjectives. Perhaps you can see a link between them that I can't see?
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for versed that I've seen before include "Knowing much" , "Accomplished" , "Proficient" , "Experienced or knowledgeable" , "Is well informed" .)