Warning when first couple have left room (5)
I believe the answer is:
amber
'warning' is the definition.
(amber traffic light)
I cannot understand how the remainder of the clue works.
(Other definitions for amber that I've seen before include "Light giving warning" , "gives warning" , "Fossil resin - warning light" , "Fossil resin used in jewellery" , "Neither red nor green?" .)