Nautical units (5)
I believe the answer is:
knots
'nautical units' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for knots that I've seen before include "They secure" , "the going rate, in the main" , "Looped fastenings" , "Grannies, reeff etc" , "Nautical mph" .)