Injection sent back in same needle (5)
I believe the answer is:
enema
'injection' is the definition.
(I've seen this before)
'sent back in same needle' is the wordplay.
'sent back' says the letters should be written in reverse.
'in' indicates a hidden word.
'amene' is hidden in the letters of 'same needle'.
'amene' back-to-front is 'ENEMA'.
(Other definitions for enema that I've seen before include "process to ensure evacuation" , "Medication introduced from behind" , "Colonic irrigation" , "medicine to get you going" , "Procedure for vacating the bowels" .)