A dreadful barber’s not in agreement (2,4)
I believe the answer is:
'not in agreement' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for at odds that I've seen before include "In disagreement or conflict - like bookies?" , "In conflict with, oppposed to (2,4)" , "Conflicting (with)" , "Opposed, in conflict with" , "Quarrelling" .)