Badminton competition's opening by end of June (5)
I believe the answer is:
event
'badminton competition's' is the definition.
'opening by end of june' is the wordplay.
'opening' becomes 'vent' (vent is a kind of opening**).
'by' means one lot of letters go next to another.
'end of' says to take the final letters.
The final letter of 'june' is 'e'.
'vent' after 'e' is 'EVENT'.
(Other definitions for event that I've seen before include "tournament" , "Planned public or social occasion" , "Fixture" , "Something taking place" , "High jump, say" .)