Spool for thread endlessly going up and down (6)
I believe the answer is:
bobbin
'spool for thread' is the definition.
(cylinder which thread is spooled round)
'endlessly going up and down' is the wordplay.
'endlessly' means to remove the last letter.
'going up and down' becomes 'bobbing' (I've seen this before).
'bobbing' with its final letter removed is 'BOBBIN'.
(Other definitions for bobbin that I've seen before include "Spindle for holding thread" , "Cylinder for winding thread" , "Reel for yarn" , "Cotton reel" , "Shuttle" .)