I don't think there is a universally accepted phrase in Hebrew, but following this definition:
semi-retired
■ adjective having retired from employment or an occupation but continuing to work part-time or occasionally.
I think a good option would be:
"גמלאי במשרה-חלקית"
or
"גמלאית במשרה-חלקית"
for women



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Note added at 14 mins (2009-08-04 18:00:44 GMT)
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note that this translation is a noun while the source is an adjective, but I imagine both can be used in semilar contexts.