Told to keep a diary of a Beat exponent (9)
I believe the answer is:
logarithm
'exponent' is the definition.
(I know that logarithm is a type of exponent)
'told to keep a diary of a beat' is the wordplay.
I cannot really understand how this works, but
'a' is within the answer.
'diary' could be 'log' (log can mean a record or diary) and 'log' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for logarithm that I've seen before include "Power to which base number must be raised to obtain a given number" , "Exponent" , "Mathematical aid simplifying multiplication and division - a girl moth (anag)" , "It undoes power" , "Aid to mathematical calculation" .)