This is alfa romeo giulietta, in front of the throttle body.
You just undo it with a suitable size spanner or socket, probably 6, 7 or 8mm. It looks to be one of the few without a screwdriver slot or phillips screw head.
If it is rounded or seized, I have found the most time-effective solution is to "crush" the screw-locking mechanism which releases the strap... Trying to lever up the strap to get cutters etc in the gap usually damages the rubber.
What I have used that seems to work well is if you have access to a cut-off tool or similar tool is to cut a straight slot in the top of the head so a flat blade screw driver can be used to loosen. This way you do not damage the head and it can be reused.
It seen that is rounded, try to use a normal plier, if that does't work use a vise grip plier (not sure if it is call like that) if it is really stuck, spray some penetrating fluid you can use wd-40 but it isn't the best choice, or try to aply some heat, just be carefull with plastic parts and rubber hoses