Maintain a grasp (4,2)
I believe the answer is:
hold on
(Other definitions for hold on that I've seen before include "Retain grasp, and don't ring off" , "Cling to - just a second" , "Cling to or informally stop a mo'" , "Informally, just a mo', or don't let go" , "Keep one's grasp or maintain telephone contact" .)