Prepare to shoot excellent Frenchman (3)
I believe the answer is:
aim
'prepare to shoot' is the definition.
'excellent frenchman' is the wordplay.
'excellent' becomes 'ai'.
'frenchman' becomes 'm' (abbreviation for Monsieur - like 'Mr').
'ai'+'m'='AIM'
(Other definitions for aim that I've seen before include "Train" , "Point or direct, a weapon say" , "Direct; intention" , "End" , "Set sights on, target or goal" .)