Very best, most favourable (7)
I believe the answer is:
optimum
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">very best</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">most favourable</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for optimum that I've seen before include "Best, most favourable" , "top condition" , "Most appropriate" , "Most favourable amount" , "Most favourable point" .)