Before Ed joined one (4)
I believe the answer is:
unit
'one' is the definition.
(synonyms)
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 "- - trust" , "Single undivided entity" , "Single manufactured item; military grouping" , "Single whole as part of a larger whole" , "Self-contained part" .)