After competition's opening, enter jam (4)
I believe the answer is:
clog
'jam' is the definition.
(clogging is a kind of jamming)
'after competition's opening enter' is the wordplay.
'after' is a charade indicator (letters next to each other).
'opening' indicates taking the first letters.
'enter' becomes 'log' (logging is a kind of entering).
The initial letter of 'competitions' is 'c'.
'c' next to 'log' is 'CLOG'.
(Other definitions for clog that I've seen before include "Footwear - hamper" , "Shoe with a thick wooden sole" , "Dutch shoe" , "Wooden footwear" , "Block up - type of shoe" .)