Striving to pull out fourth thread (6)
I believe the answer is:
string
'thread' is the definition.
(I know that thread can be written as string)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for string that I've seen before include "Cord for a group of racehorses" , "A lightweight cord" , "Procession" , "A - - of Pearls" , "Violin wire" .)