Dog originally in good health is to live (5)
I believe the answer is:
dwell
'is to live' is the definition.
(to live/dwell somewhere)
'dog originally in good health' is the wordplay.
'originally' indicates taking the first letters.
'in good health' becomes 'well' (similar in meaning).
The first letter of 'dog' is 'd'.
'd'+'well'='DWELL'
(Other definitions for dwell that I've seen before include "Live (in a place)" , "Settle" , "Hang out" , "Inhabit" , "Lodge" .)