Blow using fists' digits (3-3)
I believe the answer is:
one-two
'blow using fists' digits' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for one-two that I've seen before include "Pair of blows in boxing" , "Swift couple of blows at boxing" , "Boxing or soccer manoeuvre" , "Pair of punches with both hands in quick succession" , "Blow using fists' digits" .)