Not fat (4)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">not fat</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know this)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for thin that I've seen before include "Slender, narrow" , "(Of smile) weak, forced" , "Skinny" , "Carrying little fat" , "With little thickness" .)
(Other definitions for lean that I've seen before include "Sir David - - (film director)" , "'Tend, incline (4)'" , "Tend towards being thin" , "Slight - incline" , "Of meat, with little or no fat" .)