Box heavyweight after initially cutting and running (6)
I believe the answer is:
carton
'box' is the definition.
(carton is a kind of box)
'heavyweight after initially cutting and running' is the wordplay.
'heavyweight' becomes 'ton' (I've seen this before).
'after' says to put letters next to each other.
'initially' says to take the initial letters.
The initial letters of 'cutting and running' is 'car'.
'ton' after 'car' is 'CARTON'.
(Other definitions for carton that I've seen before include "Cantor (anag)" , "Dickensian hero" , "Container for food or liquid" , "Box made of cardboard" , "He did a far, far better thing" .)