Study after the unfinished yarn (6)
I believe the answer is:
thread
'yarn' is the definition.
('thread' can be a synonym of 'yarn')
'study after the unfinished' is the wordplay.
'study' becomes 'read' ('read' can be a synonym of 'study').
'after' says to put letters next to each other.
'unfinished' means to remove the last letter.
'the' with its final letter taken off is 'th'.
'read' put after 'th' is 'THREAD'.
(Other definitions for thread that I've seen before include "Fine cord of twisted fibres" , "Line of cotton - plot (of book, film)" , "Twine" , "Strand for sewing" , "The rib going around a screw" .)