I understand that legally a Council has to have a Clerk and at this time, the Deputy Clerk is covering this legal requirement. If the Deputy Clerk is not interested / able to take on the role permanently, I would envisage that they should be called the "Interim Town Clerk" until the position is filled. Recently in this Borough, the s151 officer resigned and another member of staff became the Interim s151 officer until another person was appointed. As for the grade / salary, that would be for the council and Deputy Clerk to agree.
The Deputy Town Clerk can still call meetings and deal with the administration, what is more of a concern is the RFO side of the role and who is dealing with that at the moment. There may need to be a formal appointment agreed by the Council asap, especially with the time of year and precepting and budgets...