Ruthless interrogation for Ph.D.? (5,6)

I believe the answer is:

third degree

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'ruthless interrogation' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for third degree that I've seen before include "Ruthless interrogation" , "doctorate, possibly?" , "What you may study for after master's" , "Relentless questioning" , "Demanding quiz" .)

I've seen this clue in The Everyman puzzle in The Observer.

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