Affirmative start to yesterday (3)
I believe the answer is:
yes
'affirmative' is the definition.
(affirmative answer word)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for yes that I've seen before include "Affirmative response" , "Pro vote" , "Indeed" , "Agreed" , "Yea" .)