Can i substitute left over (flat) sparkling wine for dry white - i'm steaming clams with butter, garlic, etc...

If it's a dry sparkling wine, yes, absolutely. Be careful however with any sweet sparklies--Asti spumante is notoriously sweet. Any champagne not categorized as "brut" will be on the sweet side. Delicious on their own, these wines will clash quite horribly with clams, garlic, etc.