Charge per unit (4)
I believe the answer is:
rate
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">charge per unit</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for rate that I've seen before include "Be worthy of; speed" , "The price paid" , "Find fault with - speed - charge" , "Amount measures against another" , "Assign a value to something" .)