Signal to proceed (3)
'signal to proceed' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for green light that I've seen before include "Signal of consent" , "Go-ahead" , "OK" , "something to go on" , "What one has to go on" .)
(Other definitions for nod that I've seen before include "Indicate assent or recognition" , "Move head in agreement" , "Tacit agreement" , "Drowse" , "Cain left Eden for the Land of . . ." .)