Father of Prince? (3)

I believe the answer is:

rex

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'father of prince?' is the definition.
The definition suggests a singular noun which matches the answer.

This is the entire clue.

(Other definitions for rex that I've seen before include "King - Harrison?" , "Oedipus - -" , "King (L.)" , "See 6" , "King (Latin); cat breed" .)

I've seen this clue in the Evening Standard.

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