Possible dearth of what a seamstress needs (6)

I believe the answer is:

thread

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

'possible' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'dearth of what a seamstress needs' is the wordplay.
'dearth' can be anagrammed to 'THREAD'.
But, I'm unsure how this is indicated.

Can you help me to learn more?

(Other definitions for thread that I've seen before include "Subject" , "'Strand of cotton, silk etc (6)'" , "Yarn, string" , "Strand of yarn" , "clothes casually cut" .)

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.