500 in good health remain (5)
I believe the answer is:
dwell
'remain' is the definition.
(I've seen this before)
'500 in good health' is the wordplay.
'500' becomes 'd' ('d' can be a synonym of '500').
'in good health' becomes 'well' ('well' can be similar in meaning to 'in good health').
'd'+'well'='DWELL'
(Other definitions for dwell that I've seen before include "Remain" , "Exist (within)" , "Inhabit" , "Lodge" , "Think moodily (on something)" .)