Give the lie to root in U-turn (5)
I believe the answer is:
rebut
This clue could be a double definition.
'give the lie to' is the first definition.
(I've seen this before)
'root in u-turn' is the second definition.
Although both the answer and definition are verbs in their base form, I cannot see how one could define the other.
(Other definitions for rebut that I've seen before include "Prove (something) to be false" , "Prove to be false or incorrect" , "Contradict or disprove" , "Disprove an accusation" , "Claim or prove to be false" .)