Ship's doctor, where child needs amputation, is prompt (4)
I believe the answer is:
urge
'is prompt' is the definition.
(thesaurus)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(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" .)