Remain healthy after fourth grade (5)
I believe the answer is:
dwell
'remain' is the definition.
(I've seen this before)
'healthy after fourth grade' is the wordplay.
'healthy' becomes 'well' (associated in meaning).
'after' means one lot of letters go next to another.
'fourth grade' becomes 'd' (fourth best grade after A, B and C).
'well' after 'd' is 'DWELL'.
(Other definitions for dwell that I've seen before include "Hang out" , "Focus attention (on)" , "Lodge" , "Think moodily (on something)" , "Abide, reside" .)