Question

Clerk have gone off sick but is using a councillor as his agent. Can this legal

Answer 1

Depends on what the reason for sickness is

Answer 2

His agent for what?

Answer 3

If the clerk is sick then the clerk should be recovering from their sickness not dealing with work matters nor involved in the delegation of their work because they're sick. They really should be focusing on recovering.
You should have a deputy to cover if a short term absence or, short to meduim term, perhaps the deputy steps up with a pay rise due to more responsibility or a locum clerk be found to help until the clerk returns.

Answer 4

We need to know what the Councillor is serving as an agent for