Stay behind (6)
'stay behind' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for shadow that I've seen before include "Inseparable follower" , "Secretly tail someone" , "Hint, remnant; follow secretly" , "It's cast by something blocking light" , "Area of darkness caused by obstructing light" .)
'stay behind' is the definition.
(remain can mean to stay behind)
This is the entire clue.
(Other definitions for remain that I've seen before include "Survive" , "Continue in position" , "Stay (in a place)" , "Stay (in position)" , "Stay on after others have gone" .)