Mend a sock (4)
I believe the answer is:
darn
'mend a sock' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for darn that I've seen before include "Repair holes by sewing" , "Stitch" , "Mend with wool using needles" , "Mend with stitches" , "Repair (sock)" .)