@musicfreak12 I think it’s supposed to be her getting turned on, but presented with a sound. Like if she was a guy it might say “pop” or “boing.” I’ve seen “clench” used this way in manga before and suspect that something is getting lost in translation. For what it’s worth, I consulted with a lady friend and she reports that “clench” is definitely weird and that “tingle” would be better.
This is Nagisa's debut work. She did do a one off chapter for an anthology a long while back, and she's starting up a second series by the looks of things, but being new to the industry she really has no other work for you to indulge in.
@musicfreak12: Likely she got a bit too moist when she clenched up her muscles, hence her then reaching for a tissue.
It was probably due to Mako's own embarrassment that she didn't read about the features of the toy, and just grabbed and paid for whatever item seemed basically similar to the Dragon.
The vibe is giving Mako a continuous, gentle sensation, on her clit, based on its position in the panel where she turns it on. This is much different from the intense and somewhat painful experience she gets from Shinichi's passionate jabbing into her "abalone". I said this before, what she needs is a gentler performance from Shinichi; and all that goes well will cum well.