Extremely healthy all right! (4,4)
I believe the answer is:
very well
I believe this clue is a double definition.
'extremely healthy' is the first definition.
(I've seen this before)
'all right' is the second definition.
(synonyms)
(Other definitions for very well that I've seen before include "Phrase indicating compliance or assent" , "If you insist" , "if you want" , "In excellent health - all right" , "All right, an expression of consent" .)