Lewis structures of molecules containing oxygen
If you consider the $\ce{ClO2-}$ as coming together from neutral atoms (plus the extra electron), all the bonding electrons "come from" the chlorine. This is commonly called a 'dative covalent bond' (or just 'dative bond' for short), and is sometimes drawn like this, instead of just a line:



As this image suggests, you can think of it as an atom with a lone pair bonding with an atom with no spare valence electron.