Blade finally opens a series of letters? (5)
I believe the answer is:
sword
'blade' is the definition.
(I've seen this before)
'finally opens a series of letters?' is the wordplay.
'finally' suggests the final letters.
'a 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 "killer?" , "Sharp-edged weapon" , "Foil" , "Weapon of steel" , "Fencer's weapon" .)