Mend with stitches (4)
I believe the answer is:
'mend with stitches' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for darn that I've seen before include "Mild oath; repair (socks)" , "Stitch" , "Mend with stitches" , "Repair (sock)" , "Repair by sewing, socks perhaps" .)