Fitter (9)
I believe the answer is:
healthier
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fitter</span>' is the definition.<br/ ><span class="explanation-format__explanation">('healthy' is associated in meaning with 'fit')</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for healthier that I've seen before include "More in the pink" , "Fitter" , "less prone to disease" .)