The drift of a yarn (6)
I believe the answer is:
thread
I believe this is a double definition.
'the drift' is the first definition.
(thesaurus)
'yarn' is the second definition.
(I know that thread is a type of yarn)
'of a' acts as a link.
(Other definitions for thread that I've seen before include "Sewing chord" , "Twine" , "The rib going around a screw" , "Fine cord of twisted fibers" , "Hatred (anag)" .)