Triad shot in the arm (5)
I believe the answer is:
tonic
This clue is a double definition.
'triad' is the first definition.
(thesaurus)
'shot in the arm' is the second definition.
(both can mean something which stimulates)
(Other definitions for tonic that I've seen before include "Invigorating liquid" , "Medical restorative" , "Gin mixer - pick-me-up" , "medicinal preparation" , "__water; gin mixer" .)