Valency is the combining power of an element.
Univalent means having valency of one. Here chlorine has valency of one and can make only one bond.
If we check the structure of chlorine, it has 7 electrons in its outermost shell. And if we follow the octate rule, chlorine will try to attain 8 electrons in its outermost shell and get stability (inert configuration). So, it will require only one electron to attain that configuration. As no. of gaining or losing electron determines valency of an element,chlorine will have valency one.
Negative sign with one (-1) indicates that the chlorine will attain its stable configuration(inert configuration) by gaining one electron. While plus sign with seven(+7) indicates that it will lose 7 electron to attain the stability. Gaining of one electron is very easy as compare to losing 7 electron. That's why chlorine has -1 valency not +7.