Party supporting Johnson ultimately, in principle — this heavyweight must be fired (10)
I believe the answer is:
cannonball
'this heavyweight must be fired' is the definition.
'party supporting johnson ultimately in principle' is the wordplay.
'party' becomes 'ball' (ball is a kind of party).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'ultimately' suggests the final letters.
'in' means one lot of letters goes inside another.
'principle' becomes 'canon'.
The final letter of 'johnson' is 'n'.
'n' put inside 'canon' is 'cannon'.
'ball' put after 'cannon' is 'CANNONBALL'.
(Other definitions for cannonball that I've seen before include "Big shot" , "Battlefield projectile" , "Old projectile" , "one fired long ago" .)