What wallet did you use to create the 1DCVZer3a9h2oWCHj778e9zWatseehD69g address?
If you have access to the private key (for example, if you used Bitcoin Core or Electrum to create that address), then you can sign and broadcast the following transaction. This will have a chance of moving the bitcoins out of the thief's reach:
0200000001a8416eb51ec474492de237e7d6c7c8d181e4480fd49004a6efcc63b89e3aba7e0200000000ffffffff0140d40d1f000000001976a91433330dc0339251333874c166688faa40dc0698bc88ac00000000
That transaction will pay a transaction fee of 0.01995866 BTC to the miners to encourage them to confirm that transaction INSTEAD of the thief's transaction. I can change that fee to any amount you prefer before you sign it, but keep in mind that the higher the fee, the more likely that we can get a miner to accept the replacement transaction instead of the thief's.
It will move the remaining 5.21 BTC to the address 15fid4LjGXFpVv8erZA2HcvqgQTcrXyjgk, which is a secure address where I have exclusive access to the private key.
If this replacement transaction confirms, I'll happily forward the 5.21 BTC (minus a small transaction fee for the miner) to any address that you feel is secure.
do you know how a transaction can be sent out without first being confirmed?
It is easy to do. Happens all the time. As you can see above, I've just created a transaction that spends those same bitcoins. Sending a transaction does not require the funding transaction to confirm.
a. Have I been hacked and are my will my BTC be stolen?
Possibly. Possibly not. That depends on what wallet you are using.
If you are using an account at a service (such as at Coinbase.com), then that second transaction could just be the service moving the bitcoins to cold storage. If you are using a software wallet that runs on your own computer, then you probably have malware on your computer that stole the private key as soon as you created the address.
b. If so can someone explain how a transaction for 5.xx BTC can be initiated from an empty wallet/address (there were no BTC at that address when I sent them from the exchange, and the first transaction has not yet been confirmed)
Spending bitcoins doesn't require them to be confirmed.
c. Is it possible to cancel this transaction as it is the second one in the list, and the first one (approved one) is yet to confirm.
It is not possible to cancel a transaction. It is possible to create a competing transaction that has a good chance of being confirmed. If the competing transaction confirms then the original will become invalid and will disappear.
d. If my BTC has been stolen, can I find out who owns the address and ask them for my BTC back (wishful thinking)
You can search your computer for malware to determine how the thief gained access to your private keys. It is unlikely that you'll be able to determine what thief created the malware.