All right - really healthy (4,4)
I believe the answer is:
very well
'all right' is the definition.
(synonyms)
'really healthy' is the wordplay.
'really' becomes 'very' (I've seen this in other clues).
'healthy' becomes 'well' (associated in meaning).
'very'+'well'='VERY WELL'
(Other definitions for very well that I've seen before include "Better than just OK" , "Outstandingly" , "Enjoying great health" , "Phrase indicating compliance or assent" , "I agree" .)