Go for an injection (4)
I believe the answer is:
shot
I believe this is a double definition.
'go' is the first definition.
(thesaurus)
'an injection' is the second definition.
('shot' can be a synonym of 'injection')
'for' is the link.
(Other definitions for shot that I've seen before include "Fired; (be) rid (of)" , "Hit by gun" , "Exhausted" , "Snap" , "Fired a gun" .)