Sewing material (6)
(Other definitions for thread that I've seen before include "Subject" , "Connecting line" , "Strand of, eg, cotton" , "'Strand of cotton, silk etc (6)'" , "What sewer takes" .)
(Other definitions for yarn that I've seen before include "Rambling story" , "Tale; weave" , "Rambling tale" , "Fabricated" , "that'll keep you in stitches?" .)