Hello friends i am much worried ...
2 days ago one exchanger send me btc but still not confirmed.Few minutes ago my transaction had reversed back to exchanger account.
what is a double spending attack or not?
and how payment reversed.and now a time double spending attack is possible?
By exchanger you mean someone who traded with you not bitcoin exchange? You should notice that earlier before finishing the deal.
Never send anything first, always wait for at least 2 confirmations. Or you can use escrow service when dealing with random people from the internet.
Pity that you didn't include more details like transaction ID or the name of that individual who scammed you.