Bust leader of assassins in French port (6)
I believe the answer is:
breast
'bust' is the definition.
(I've seen this before)
'leader of assassins in french port' is the wordplay.
'leader of' suggests taking the first letters.
'in' means one lot of letters goes inside another.
'french port' becomes 'brest' (port city in Brittany).
The first letter of 'assassins' is 'a'.
'a' placed within 'brest' is 'BREAST'.
(Other definitions for breast that I've seen before include "Invalid's helper" , "Upper front part of the human torso" , "Milk producer" , "Chicken portion" , "joint for diners" .)