Opening contest shunned by beginner (4)
I believe the answer is:
vent
'opening' is the definition.
(vent is a kind of opening)
'contest shunned by beginner' is the wordplay.
'contest' becomes 'event' (I've seen this before).
'shunned by beginner' means to remove the first letter.
'event' with its first letter taken away is 'VENT'.
(Other definitions for vent that I've seen before include "Opening of a volcano" , "Slit in the back of a jacket" , "Give outlet as to feelings" , "Opening for air" , "Hole allowing gas or air to escape" .)