Weapon is back, returned having arrested foreign character (6)
I believe the answer is:
rapier
'weapon' is the definition.
(rapier is a kind of weapon)
'back returned having arrested foreign character' is the wordplay.
'back' becomes 'rear' (something's rear is its back).
'returned' says the letters should be written backwards.
'having' is an insertion indicator.
'arrested foreign character' becomes 'pi' (I can't explain this - if you can you should believe this answer much more).
'rear' back-to-front is 'raer'.
'raer' placed around 'pi' is 'RAPIER'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for rapier that I've seen before include "fencing equipment" , "Thrusting weapon" , "Fine-bladed sword" , "Long slender sword" , "Straight narrow sword with two edges" .)