Opening for a coin (4)
I believe the answer is:
slot
'opening for a coin' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for slot that I've seen before include "Opening for coin or TV programme" , "Scheduled time -- aperture for coins" , "Scheduled time - hierarchical position" , "Long narrow opening" , "Space for inserting coin" .)