Chamber opening? (4)
I believe the answer is:
ante
'chamber opening?' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ante that I've seen before include "Initial contribution to the common pot" , "'Behind, before (4)'" , "Player's stake in poker - a ten? (anag)" , "early payment" , "Stake, bet" .)