Blade glowing when lifted in fight (8)
I believe the answer is:
stiletto
'blade' is the definition.
(I have seen 'Thin blade ' mean 'stiletto' so perhaps 'blade' could also mean 'stiletto')
'glowing when lifted in fight' is the wordplay.
'glowing' becomes 'lit' (I've seen this in another clue).
'when lifted' shows that the letters should be reversed in order (in a down clue, letters go up).
'in' indicates putting letters inside.
'fight' becomes 'setto' (set-to can mean a fight or argument).
'lit' reversed gives 'til'.
'til' placed into 'setto' is 'STILETTO'.
(Other definitions for stiletto that I've seen before include "Bedspread" , "Long dagger" , "Thin blade" , "One may have a stab" , "Small dagger and high narrow heel" .)