Slangy affirmative (4)
'slangy affirmative' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for yeah that I've seen before include "Definitely (informal)" , "Yes (informally)" , "'She loves you, ..., ..., ...' (song)" , "positive response?" , "Exclamation of agreement (or doubt!)" .)
(Other definitions for sure that I've seen before include "Definite" , "The old sewer" , "of course" , "stable" , "Convinced" .)