Not having become a member of a union (6)
I believe the answer is:
single
'union' is the definition.
The answer and definition can be both acts as well as being singular nouns.
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 single that I've seen before include "Free" , "Not in a relationship" , "Only one; record" , "Individual - unmarried" , "Like bachelor" .)