A particularly filthy and/or sweaty vagina.
Could also refer to a Jammy Dodger that has been sitting in a warm lunchbox for too long, causing it to sweat.
Guy1: "Wow she's a bit of alright!"
Guy2: "Don't go there man, she's got a Clammy Dodger"
Guy1: "Ah mate, what the hell is that moist red blob in your lunchbox"
Guy2: "Clammy Dodger"