Blade finally opens series of letters? (5)
I believe the answer is:
sword
'blade' is the definition.
(I know that blade can be written as sword)
'finally opens series of letters?' is the wordplay.
'finally' indicates one should take the final letters.
'series of letters?' becomes 'word'.
The final letter of 'opens' is 's'.
's'+'word'='SWORD'
(Other definitions for sword that I've seen before include "used for fencing" , "killer?" , "Performer whose throat is in danger?" , "Normandy beach" , "Command; social class" .)