Greek character stops behind raised weapon (6)
I believe the answer is:
rapier
'weapon' is the definition.
(rapier is a kind of weapon)
'greek character stops behind raised' is the wordplay.
'greek character' becomes 'pi' (letter from the Greek alphabet).
'stops' means one lot of letters goes inside another (stopping can mean blocking a gap).
'behind' becomes 'rear' (both can mean one's bottom).
'raised' says the letters should be written in reverse (going up in a down clue).
'rear' reversed gives 'raer'.
'pi' going into 'raer' is 'RAPIER'.
(Other definitions for rapier that I've seen before include "Light sword" , "A small spring flower" , "fencing equipment" , "A thin double-edged sword" , "Narrow sword with two-edged blade" .)