Summary says everything. Something like fill="url(def.svg#grad)" is not supported.
Created attachment 50628 [details] def.svg with gradient definition
Created attachment 50629 [details] Test-case for reported bug, uses definition from first attachment
SVG image (second attachment) should be green.
Confirmed with <use xlink:href="./file.svg#whatever"> Possibly a universal external file bug?
Duplicate of https://bugs.webkit.org/show_bug.cgi?id=12499
*** This bug has been marked as a duplicate of bug 12499 ***