Piggy bank opening (4)
I believe the answer is:
slot
'piggy bank opening' 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 "Lots (anag.)" , "This gaming machine operates by insertion of coin" , "Scheduled time - hierarchical position" , "Hole for money" , "Narrow opening, for cash say" .)