Surprise opening sale with initial 50 per cent reduction (7)
I believe the answer is:
startle
'surprise' is the definition.
(to startle someone is to surprise or shock them)
'opening sale with initial 50 per cent reduction' is the wordplay.
'opening' becomes 'start' (both can mean the beginning).
'with initial t0 per cent reduction' indicates taking half.
'sale' halved is 'le'.
'start'+'le'='STARTLE'
(Other definitions for startle that I've seen before include "Alarm suddenly" , "Make jump" , "Surprise with starlet" , "Rattles (anag.)" , "Alarm (someone)" .)