No, messenger can not conceal summons from king because orders from the king are supreme.
Step-by-step explanation:
Conceal means to keep secret of something. It is hiding of something. Summon is an order given by a higher authority, in this case it is an order given by a king.
A messenger who has been instructed to pass the order of the king has no rights to conceal it or to keep it hiding and secretly from the people for whom the order has been passed. The order of the king is supreme and has to be agreed by everyone.