Not heavy (5)
I believe the answer is:
light
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">not heavy</span>' is the definition.<br/ ><span class="explanation-format__explanation">('light' is the opposite to 'heavy')</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for light that I've seen before include "Easy to digest; flame" , "Weightless; lamp" , "Portable" , "Illuminate or ignite" , "Pre-eminent person; window" .)