Regatta units (5)
I believe the answer is:
knots
'regatta 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 "the going rate, in the main" , "Tangles" , "They secure" , "Grannies, reeff etc" , "Nautical mph" .)