Clever swimmer worked on DPhil (7)

I believe the answer is:

dolphin

(Other definitions for dolphin that I've seen before include "Sea mammal like large porpoise, friendly towards people" , "Sociable sea mammal, sacred in ancient Greece" , "Sea creature" , "Marine mammal with an elongated beak-like head" , "Grampus" .)

I've seen this clue in The Sun.

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