Unhappy with it in what way ? Unless there is something wrong with the actual track (e.g. the quality of the recording, it's cut short etc) then as purchases are considered final you are unlikely to be receive a credit for it.
If there is something wrong with it then depending upon what country that you are in (music can't be re-downloaded in all countries) then try deleting the incomplete track from your iTunes library and redownload it via the Purchased link under Quick Links on the right-hand side of the iTunes store home page on your computer's iTunes : re-downloading.
If you aren't in a country where you can re-download music or if it re-downloads in the same state then try the 'report a problem' link from your purchase history : log into your account on your computer's iTunes via Store > View My Account (Store > View My Apple ID on iTunes 11) and you should then see a Purchase History section with a 'see all' link to the right of it ; click on that and you should see a list of your purchases ; find that song and use the 'Report a Problem' link and fill in details about the problem (iTunes support should reply within, I think, 24 to 48 hours).
Some people have had a problem with the 'report a problem' link (it's been taking people to this site on a browser instead of showing a form in iTunes) - if it does that to you then try contacting iTunes support via this page : http://www.apple.com/support/itunes/contact/- click on Contact iTunes Store Support on the right-hand side of the page.
I downloaded a track (little derek, by Sway) today from a compilation, the only version of the track available on iTunes, but is has a load of chat over the first part of the song, I can't even hear the intro (a part of the track I love). This was not obvious from the preview so I am disappointed. Can I get a refund on that basis?
You could try re-downloading it if you are in a country where music can be re-downloaded to see if it was a problem with the download, otherwise try contacting iTunes Support via the 'report a problem' link as described in my post above - only they will be able to confirm whether you can get a refund or credit.