Crossword Genius

Before Ed joined one (4)


I believe the answer is:

I'm a little stuck... Click here to teach me more about this clue!

'one' is the definition.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for unit that I've seen before include "Army division", "Military team", "Single element; one", "A single self-contained item", "Part of a kit".)