Union member swapped sides causing depression? (5)

I believe the answer is:

gloom

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

'depression?' is the definition.
(I've seen this in another clue)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for gloom that I've seen before include "Melancholy state" , "Darkness; depression" , "Near-darkness - misery" , "Depression and melancholy" , "Murkiness" .)

I've seen this clue in the Sydney Morning Herald.

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.