Very best (4)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">very best</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for optimum that I've seen before include "Most favourable (conditions)" , "The best possible" , "Most favourable amount" , "model" , "Most appropriate" .)