One is a 70's BBC children show where a family of Clangers lived on small moon with the soup dragon, the iron chicken and the froglets et al. When they spoke, they sounded like someone playing a swanee whistle. They ate blue string pudding.
The second definition is that one has made a mistake and "dropped a clanger". This is Cockney, not Mockney by the way.
Auntie Clanger: WoOowowowoOwOOOowoOOwooo wooo OOoo?
Baby Clanger: OoO.
Other Clangers: OOOOoooo.
Bloke 1: Shit.
Bloke 2: What?
Bloke 1: Forgot the whife's birthday.
Bloke 2: Dropped a right fucking clanger there, mate.