Sounds like the council has lost (or never had) a proper appreciation of budget setting & monitoring and possibly doesn’t have adequate ‘control’ of staff possibly combined with a clerk over stepping the mark and taking advantage of a gullible management structure.
Staff costs should be set at budget and monitored throughout the year.
Whether - or not - clerk has delegation to authorise overtime should be within a delegation scheme.
Staff duties and tasking should be in JD & TOR.
If overtime has become a regular, rather than an exceptional, circumstance then either tasking or performance are inappropriate.
If you have to ask the question - should the clerk…. Then it is most likely a systemic failure or inadequacy in council procedure.