Exponent of record with a beat you can hear (9)
I believe the answer is:
logarithm
'exponent' is the definition.
(logarithm is a kind of exponent)
'record with a beat you can hear' is the wordplay.
I cannot really understand how this works, but
'record' could be 'log' (logging is a kind of recording) and 'log' is present in the answer.
'a' is found in the answer.
This explanation may well be incorrect...
'of' is the link.
(Other definitions for logarithm that I've seen before include "It undoes power" , "Exponent" , "Mathematical aid simplifying multiplication and division - a girl moth (anag)" , "Aid to mathematical calculation" , "Power to which base number must be raised to obtain a given number" .)