Persevere ruthlessly (4,2,7)

I believe the answer is:

stop at nothing

(Other definitions for stop at nothing that I've seen before include "Ignore all scruples" , "Be totally ruthless" , "as announcers of rocket launches do?" , "3 [COUNTDOWN CONUMDRUM] will" , "Act ruthlessly" .)

I've seen this clue in The Sunday Times and The Times.

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