Start of a one-two (3)
I believe the answer is:
jab
'start of a one-two' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for jab that I've seen before include "Short straight punch" , "Poke roughly or inject" , "Inoculation" , "Prod" , "Short stiff blow or thrust" .)