Short answer
It depends on its condition. A slightly dried-out segment is usually fine, but if it was mouldy or fermented, it could cause an upset stomach. Watch for vomiting, diarrhea, or loss of appetite over the next day. Offer your dog some water and call your vet if you see any of those signs.
Clementine — Food safety facts
Feed with care · Dogs · full breakdown on the linked food page
Flesh only, a segment or two. Peel, pith and seeds are the problem.
The full verdict, portion guidance and serving notes are collected on the Clementine food facts page.