Remain behind
(Other definitions for lag that I've seen before include "Prisoner" , "trail" , "Drag one's feet" , "person inside" , "Hang back" .)
'remain behind' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for stay that I've seen before include "Check" , "Visit (for the night)" , "Rope to brace an upright" , "Arrest" , "Stanchion" .)
'remain behind' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for hang back that I've seen before include "Show reluctance" , "Remain behind" , "Get behind" .)