Characters participating in round of Open (4)
I believe the answer is:
undo
'open' is the definition.
(I've seen this before)
'characters participating in round of' is the wordplay.
'characters participating in' says the answer is hidden in the clue (characters or letters from the clue).
'UNDO' is hidden in the letters of 'round of'.
(Other definitions for undo that I've seen before include "Loosen (knot)" , "Release fastening" , "Untie or ruin" , "Take apart" , "Unfasten - destroy" .)