Wise guy (3)
(Other definitions for wag that I've seen before include "Accompanying lovely" , "Vibrate; jester" , "Person fond of tricks and playing jokes" , "Shake" , "The wit to move the tail" .)
I believe this clue is a double definition.
'wise' is the first definition.
(comedian Ernie Wise)
'guy?' is the second definition.
(I've seen this before)
(Other definitions for ernie that I've seen before include "Premium?bond computer" , "It picks Premium Bond winners" , "Guy" , "Machine that produces the winning Premium Bond numbers" , "Male" .)