Whilst all of the above comments are ‘valid’ and may be relevant, there is 1 fundamental considerations which has not yet been mentioned.
Effective staff management BY the PC.
If there is frequent overtime it MUST be authorised in advance.
In order to do that the PC must understand, DIRECT and monitor what the clerk spends their paid hrs doing.
Your council presumably authorises the overtime payments when presented and it is undeniably a RESPONSIBILITY upon Cllrs to be sure the payments are reasonable, authorised and necessary.
If overtime is dictated by the employee rather than granted by the employer your system is failing (and so is your Internal Auditor if this is not being highlighted.)
Sounds like you are trying to resolve (or at least ‘aware’ of the potential shortcoming. Proceed with caution - this is a classic scenario where clerk and associates will turn on you as a troublemaker!