Expel spear carrier on wrong diet (9)
I believe the answer is:
extradite
'expel' is the definition.
(I know that extradite is a more specific form of the action expel)
'spear carrier on wrong diet' is the wordplay.
'spear carrier' becomes 'extra' (synonyms).
'on' indicates putting letters inside.
'wrong' indicates anagramming the letters.
'diet' is an anagram of 'dite'.
'extra' placed within 'dite' is 'EXTRADITE'.
(Other definitions for extradite that I've seen before include "Send away for trial!" , "Hand over (felon) to another country" , "Hand over for trial to another jurisdiction" , "Hand over for trial in another country" , "Hand over to authorities of another country" .)