Shoelace problem (4)
I believe the answer is:
knot
'shoelace problem' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for knot that I've seen before include "Entwine" , "Shore bird; cluster" , "Ligature" , "A bowline or a reef" , "Small cluster - huddled group - sheepshank, for example" .)