Be surprised by the opening (5)
I believe the answer is:
start
'opening' is the definition.
(both can mean the beginning)
'be surprised by the' is the wordplay.
'be surprised by' becomes 'star' (I am not sure about this - if you are sure you should believe this answer much more).
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'star'+'t'='START'
Can you help me to learn more?
(Other definitions for start that I've seen before include "Take the first step" , "Outset" , "Set in motion" , "launch" , "Jump suddenly" .)