Prosecute — Vulcan is to be an informer (7)
I believe the answer is:
impeach
'prosecute' is the definition.
Although both the answer and definition are verbs in their base form, I cannot understand how one could define the other.
'vulcan is to be an informer' is the wordplay.
'vulcan is' becomes 'I'm' (where the crossword setter is 'Vulcan').
'to be an informer' becomes 'peach' (I can't justify this - if you can you should believe this answer much more).
'im'+'peach'='IMPEACH'
Can you help me to learn more?
(Other definitions for impeach that I've seen before include "Charge with a crime (against the state)" , "In America, charge with misconduct - say President" , "Charge public official with misconduct - Clinton?" , "Charge with major crime" , "Charge with treason, etc" .)