Turned by old head of a stony-faced mourner (7)

I believe the answer is:

niobean

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

'turned' is the definition.
Both the answer and definition are adjectives.
Maybe they are linked in a way I don't understand?

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for niobean that I've seen before include "very tearful" , "Weeping" , "reacting stonily?" .)

I've seen this clue in The Sunday Times and The Times.

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.