Not fast (4)
I believe the answer is:
slow
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">not fast</span>' is the definition.<br/ ><span class="explanation-format__explanation">('slow' is the opposite to 'fast')</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for slow that I've seen before include "Intellectually dull" , "Leisurely" , "lacking any precipitation" , "Lento (mus.)" , "Not quick to understand" .)