Declare an interests in what actually? The clerk carries out the wishes and instructions of the council. Which should decide the wording of the invitation to tender, the specification of the works/ item required and the list of invitees. Also dates for submission and procedure for receiving safe keeping and opening ( who will be present) of the tenders. It will be the council who decide who will be awarded the contract not the clerk.
All of this should be specified by the council and managed by the clerk. As there should be no possibility of influence by the clerk only the integrity of the clerk would guide the necessity of a declaration of interest by them.