The "one" in a one-two
I believe the answer is:
jab
'the one in 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 "A short stiff punch" , "Poke roughly or inject" , "Injection" , "Prod" , "Short straight punch" .)