Ever-changing swerve (4)
I believe the answer is:
veer
'swerve' is the definition.
(I know that swerve can be written as veer)
'ever-changing' is the wordplay.
'changing' indicates an anagram.
'ever' with letters rearranged gives 'VEER'.
(Other definitions for veer that I've seen before include "Deviate" , "Suddenly change direction" , "Turn sharply" , "Change direction or course, like wind" , "Change direction abruptly" .)