Gathering speed at sea, perhaps? (4)
I believe the answer is:
knot
I believe this is a double definition.
'gathering' is the first definition.
(thesaurus)
'speed at sea perhaps?' is the second definition.
(measure of speed once used on ships)
(Other definitions for knot that I've seen before include "Measure of speed at sea" , "measure of pace" , "securing tie" , "Tie; a cluster" , "Sort of garden" .)