Test one conservative holding back effort diversifying unions (8)

I believe the answer is:

exogamic

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

'test' is the definition.
Both the definition and answer are adjectives. Perhaps there's a link between them I don't understand?

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

I've seen this clue in The Guardian.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.