Greek character in rear returning sword (6)
I believe the answer is:
rapier
'sword' is the definition.
(sword used in fencing)
'greek character in rear returning' is the wordplay.
'greek character' becomes 'pi' (letter from the Greek alphabet).
'in' is an insertion indicator.
'returning' says the letters should be written backwards.
'rear' written backwards gives 'raer'.
'pi' inserted within 'raer' is 'RAPIER'.
(Other definitions for rapier that I've seen before include "fencing equipment" , "Thin sharp sword" , "A fencing sword for repair" , "Thrusting weapon" , "Straight sword with narrow blade" .)