"Sounds good!" (4)
'sounds good' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for okay that I've seen before include "Approve, endorse" , "Agreed!" , "Correct" , "Permissible; fairly good" , "in order" .)
(Other definitions for cool that I've seen before include "chic" , "Calm; chilly" , "Unfriendly; excellent" , "Fashionable (but don't lose yours!)" , "Sangfroid" .)
(Other definitions for yeah that I've seen before include "Absolutely!" , "Exclamation of agreement (or doubt!)" , "Correct" , "Positive (or sceptical) exclamation" , "quite casually" .)