Unduly high (5)
I believe the answer is:
steep
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">unduly high</span>' is the definition.<br/ ><span class="explanation-format__explanation">(as in a steep or expensive price)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for steep that I've seen before include "Sheer, precipitous" , "Saturate; precipitous" , "Rapidly rising" , "over the top" , "Having a sharp incline" .)