Agitate those, briefly, for injection (4)
I believe the answer is:
shot
'injection' is the definition.
(I know that injection can be written as shot)
'agitate those briefly' is the wordplay.
'agitate' is an anagram indicator (I've seen this in other clues).
'briefly' means to remove the last letter.
'those' with its final letter taken away is 'thos'.
'thos' with letters rearranged gives 'SHOT'.
'for' is the link.
(Other definitions for shot that I've seen before include "Small drink of spirits" , "Exhausted" , "glass of spirits" , "Ruined; photograph" , "Slug" .)