Veer suddenly (6)
(Other definitions for swerve that I've seen before include "Suddenly move off course" , "Turn sharply aside" , "Suddenly deviate from straight course" , "Using spin" , "Divergence from course" .)
'veer suddenly' is the definition.
(I've seen this in another clue)
This is all the clue.
(Another definition for zag that I've seen is " change of direction".)