Back up, holding good sword (6)
I believe the answer is:
rapier
'back up holding good sword' is the definition.
'rapier' can be an answer for 'sword' (rapier is a kind of sword). I am not certain of the 'back up holding good' bit.
'back up holding good sword' is the wordplay.
'back' becomes 'rear' (synonyms).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'holding' indicates putting letters inside.
'good sword' becomes 'pi' (I am not sure about this - if you are sure you should believe this answer much more).
'rear' backwards is 'raer'.
'raer' going around 'pi' is 'RAPIER'.
(Other definitions for rapier that I've seen before include "Light sword" , "A small spring flower" , "Thin sharp sword" , "Sword for thrusting" , "A thin double-edged sword" .)