They can be tied at speed (5)
I believe the answer is:
knots
'speed' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for knots that I've seen before include "Looped fastenings" , "Grannies, reeff etc" , "Tangles" , "Nautical mph" , "the going rate, in the main" .)