Swerves at sea (4)
I believe the answer is:
'swerves at sea' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for yaws that I've seen before include "Doesnt go straight; tropical disease" , "Goes off course (ship); tropical disease" , "Deviates" , "course changes" , "Moves unsteadily" .)