Not slow (4)
I believe the answer is:
fast
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">not slow</span>' is the definition.<br/ ><span class="explanation-format__explanation">('fast' is the opposite to 'slow')</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for fast that I've seen before include "Flying" , "Period without food" , "Rapid - diet" , "not lento" , "Avoid food; rapid" .)