Mathematical exponent (9)
I believe the answer is:
logarithm
'mathematical exponent' is the definition.
(I've seen this in another clue)
This is all the clue.
(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)" .)