Green light (3)
'green light' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for nod that I've seen before include "Green light" , "The Land of ... lay east of Eden" , "Move head in assent" , "It's said to be as good as a wink" , "drift" .)
'green light' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for go-ahead that I've seen before include "Up-and-coming - authorisation" , "Enterprisingly progressive" , "Permission to proceed" , "Pioneering (green light?)" , "Enterprising" .)