Extremely healthy (4,4)
I believe the answer is:
very well
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">extremely healthy</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for very well that I've seen before include "Extremely healthy - phrase signifying assent (sometimes ironically)" , "Blooming" , "Better than just OK" , "I agree" , "Enjoying great health" .)