Most costly (7)
I believe the answer is:
dearest
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">most costly</span>' is the definition.<br/ ><span class="explanation-format__explanation">('dear' can be a synonym of 'costly')</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for dearest that I've seen before include "Most expensive or beloved" , "Affectionate address" , "Most loved or expensive" , "Loved one" , "most excessive" .)