Stay behind with one kept in by staff again (6)
I believe the answer is:
remain
'stay behind' is the definition.
(remain can mean to stay behind)
'one kept in by staff again' is the wordplay.
'one' becomes 'i' (Roman numeral).
'kept in' is an insertion indicator.
'by' means one lot of letters go next to another.
'staff' becomes 'man' (as in staffing or manning a particular place).
'again' becomes 're' (prefix meaning 'again').
'man' after 're' is 'reman'.
'i' inserted inside 'reman' is 'REMAIN'.
'with' acts as a link.
(Other definitions for remain that I've seen before include "Survive" , "Don't leave (the EU?)" , "Continue in position" , "Marine (anag)" , "Stay on when others have left" .)