Clearly not a singlet (7)

I believe the answer is:

doublet

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

'singlet' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe you can see an association between them that I don't see?

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for doublet that I've seen before include "Former male, close-fitting body garment" , "Jacket once, went with hose" , "Old garment worn with hose" , "Pair of similar things" , "Formerly, a man's close-fitting jacket" .)

I've seen this clue in the Evening Standard.

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.