Formal affirmative (3)
'formal affirmative' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for yes that I've seen before include "One response to "will you marry me?"" , "Yea" , "Absolutely" , "That's 27 [RIGHT]" , "Oui" .)
(Other definitions for yea that I've seen before include "Positive vote" , "'She loves you, ... ... ...' (The Beatles)" , "Certainly (archaic)" , "Affirmative in the British Parliament" , "Yes (archaic)" .)