Kidnap a French duke in outskirts of Brest (6)
I believe the answer is:
abduct
'kidnap' is the definition.
(synonyms)
'a french duke in outskirts of brest' is the wordplay.
'french duke' becomes 'duc' ('duke' in French).
'in' indicates putting letters inside.
'outskirts of' means to remove the middle letters.
'brest' with its middle taken out is 'bt'.
'duc' inserted inside 'bt' is 'bduct'.
'a'+'bduct'='ABDUCT'
(Other definitions for abduct that I've seen before include "Carry off illegally" , "Take (someone) illegally" , "Take (someone) away by force" , "Remove by force" , "Hijack" .)