Most favourable, best (7)
I believe the answer is:
optimal
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">most favourable</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">best</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(optimal means the best)</span><br/ ><br/ ></span>
(Other definitions for optimal that I've seen before include "couldn't be better" , "Am pilot being at best" , "best results" , "Plot aim (anag) - most favourable" , "Best or most favourable" .)