To start with, Joe interrupts Ben (5)
I believe the answer is:
begin
'to start with' is the definition.
(I've seen this before)
'joe interrupts ben' is the wordplay.
'joe' becomes 'GI' (GI Joe).
'interrupts' is an insertion indicator.
'gi' put into 'ben' is 'BEGIN'.
(Other definitions for begin that I've seen before include "Initiate" , "former Israeli Nobel peace prize winner" , "Israeli prime minister who shared the Nobel Peace Prize with President Sadat" , "Monachem --" , "Commence" .)