There does not seem to be a way to change the starting timecode of a compound clip in the FCP interface, but
I found a workaround that may be helpful:
1) Select the compound clip in the browser, and do File->Export XML
2) Open the xml file (*) in a text editor (I used TextMate, but you can use BBEdit, or any that you prefer),
and change this line (should be near the top, in this case it was line 7):
<sequence format="r2" duration="70300/2500s" tcStart="0s" tcFormat="NDF" audioLayout="stereo" audioRate="48k">
to put in the desired timecode, eg:
<sequence format="r2" duration="70300/2500s" tcStart="11s" tcFormat="NDF" audioLayout="stereo" audioRate="48k">
Save the file
3) Import the XML file you just edited.
You will be asked if you want to replace the existing clip, or keep both.
I suggest keeping both, until you make sure it all works.
(*) If you choose version 1.9 of the xml, this is directly an fcpxml file; if you choose version 1.10, it is a bundle, the file extension is .fcpxmld and you have to Show Package Contents, and edit the Info.fcpxml inside the package.