Where boater may go down instantly? (2,3,4,2,1,3)
I believe the answer is:
at the drop of a hat
'instantly?' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
(Other definitions for at the drop of a hat that I've seen before include "Immediately" , "When pork pie downed" , "When a tile falls to the ground" , "instantly" , "in no time at all" .)