This is a container clue - look for indicators like 'around', 'holding', 'outside', 'embracing' where one part wraps around another
One part goes inside or around another
Answer: SASHWINDOW | Definition: 'Source of light' | Wordplay: An envelope (‘in’) of ASH (‘remains’) plus WIND (air ‘current’) in SOW (‘plant’, verb, although I would use SOW to refer to seeds but ‘plant’ to seedlings or larger). Of course, the actual source of light is outside the window, but the window introduced the illumination into a room.