Linger on second course? (6)
I believe the answer is:
remain
'linger' is the definition.
(I know that linger is a kind of remain)
'on second course?' is the wordplay.
'on' means one lot of letters go next to another.
'second' becomes 're' (prefix meaning second in eg review, re-education).
'course?' becomes 'main' (main course is a kind of course).
're' put next to 'main' is 'REMAIN'.
(Other definitions for remain that I've seen before include "Stay around" , "Stay or continue to be" , "Marine (anag)" , "Last" , "Continue in position" .)