That's right - it may be terday! (3)
I believe the answer is:
yes
'that's right' is the definition.
(right can mean yes or okay)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for yes that I've seen before include "Word of affirmation" , "Aye" , "Pro vote" , "Affirmative answer" , "That's 27 [RIGHT]" .)