Loki's hands grip tightly at Clint's hips. He isn't just holding him steady he is putting some strength into taking the strain off his arms however minutely. But if he was really concerned he would just cut him down.

Having little reaction from him isn't stopping him. He draws him in, all the way, sucks hard for a moment then pulls back. He repeats it several times before moving his attention solely to the head of his cock.