Not weak (6)
I believe the answer is:
strong
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">not weak</span>' is the definition.<br/ ><span class="explanation-format__explanation">('strong' is the opposite to 'weak')</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for strong that I've seen before include "Able to withstand attack" , "firm" , "Robust and muscular" , "Physically powerful - feel 1 Across" , "Convincing" .)