Complete reversal (5-4)
'complete reversal' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for u-turn that I've seen before include "180deg change of direction" , "Complete change of direction" , "Complete reversal of car or attitude" , "A complete reverse of direction" , "Complete reversal of direction or policy" .)
(Other definitions for about-turn that I've seen before include "Act of pivoting 180 degrees" , "Change of opinion" , "Total change of direction" .)