Initially Sheffield Wednesday overcoming rivals, duly surprising "The Blades" (6)
I believe the answer is:
swords
'the blades' is the definition.
('sword' can be a synonym of 'blade')
'initially sheffield wednesday overcoming rivals duly surprising' is the wordplay.
'initially' suggests taking the first letters.
The first letters of 'sheffield wednesday overcoming rivals duly surprising' is 'SWORDS'.
(Other definitions for swords that I've seen before include "Thrusting weapons" , "Weapons from north Co Dublin?" , "Fighting blades" , "Weaponry" , "Steel weapons" .)