Mathematical exponent (9)
I believe the answer is:
logarithm
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mathematical exponent</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for logarithm that I've seen before include "It undoes power" , "Exponent" , "Aid to mathematical calculation" , "Power to which base number must be raised to obtain a given number" , "Mathematical aid simplifying multiplication and division - a girl moth (anag)" .)