Crossword Genius

Ship's doctor, where child needs amputation, is prompt (4)

Ross

I believe the answer is:

urge

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'is prompt' is the definition.
(thesaurus)

I don't understand how the remainder of the clue works.

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(Other definitions for urge that I've seen before include "Egg on, encourage to proceed" , "Sudden inclination to do something" , "Encourage to go on, goad" , "Advise earnestly, encourage to go on" , "Impel, egg on" .)

I've seen this clue in The Telegraph.
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